The behavior of "systemctl list-units" and "systemctl list-units" when they output a service whose state is "unknown"

Issue

• The status of the service is "unknown" only when a service whose status is inactive and disabled.

[root@rhel7u2-x64 ~]# systemctl status firewalld 
● firewalld.service - firewalld - dynamic firewall daemon 
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled) 
   Active: active (running) since Tue 2017-01-17 21:25:28 JST; 17s ago 
Main PID: 26651 (firewalld) 
   CGroup: /system.slice/firewalld.service 
           mq26651 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid

Jan 17 21:25:28 rhel7u2-x64 systemd[1]: Starting firewalld - dynamic firewall daemon... 
Jan 17 21:25:28 rhel7u2-x64 systemd[1]: Started firewalld - dynamic firewall daemon. 
[root@rhel7u2-x64 ~]# systemctl is-active firewalld 
active                                                   <-- active and enabled 
[root@rhel7u2-x64 ~]# systemctl stop firewalld 
[root@rhel7u2-x64 ~]# systemctl is-active firewalld 
inactive                                                 <-- inactive and enabled 
[root@rhel7u2-x64 ~]# systemctl start firewalld 
[root@rhel7u2-x64 ~]# systemctl disable firewalld 
Removed symlink /etc/systemd/system/basic.target.wants/firewalld.service. 
Removed symlink /etc/systemd/system/dbus-org.fedoraproject.FirewallD1.service. 
[root@rhel7u2-x64 ~]# systemctl is-active firewalld 
active                                                   <-- active and disabled 
[root@rhel7u2-x64 ~]# systemctl stop firewalld 
[root@rhel7u2-x64 ~]# systemctl is-active firewalld 
unknown                                                  <-- inactive and disabled  (unknown) ###
[root@rhel7u2-x64 ~]# 

• A service whose status is "unknown" is not output by "systemctl list-units --type service --all".

[root@rhel7u2-x64 ~]# systemctl status firewalld 
● firewalld.service - firewalld - dynamic firewall daemon 
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled) 
   Active: active (running) since Tue 2017-01-17 21:28:42 JST; 5min ago 
Main PID: 27643 (firewalld) 
   CGroup: /system.slice/firewalld.service 
           mq27643 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid

Jan 17 21:28:42 rhel7u2-x64 systemd[1]: Starting firewalld - dynamic firewall daemon... 
Jan 17 21:28:42 rhel7u2-x64 systemd[1]: Started firewalld - dynamic firewall daemon. 
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep firewalld 
  firewalld.service                      loaded    active   running firewalld - dynamic firewall daemon    <-- active and enabled 
[root@rhel7u2-x64 ~]# systemctl stop firewalld 
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep firewalld 
  firewalld.service                      loaded    inactive dead    firewalld - dynamic firewall daemon    <-- inactive and enabled 
[root@rhel7u2-x64 ~]# systemctl start firewalld 
[root@rhel7u2-x64 ~]# systemctl disable firewalld 
Removed symlink /etc/systemd/system/basic.target.wants/firewalld.service. 
Removed symlink /etc/systemd/system/dbus-org.fedoraproject.FirewallD1.service. 
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep firewalld 
  firewalld.service                      loaded    active   running firewalld - dynamic firewall daemon    <-- active and disabled  
[root@rhel7u2-x64 ~]# systemctl stop firewalld 
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep firewalld                           <-- inactive and disabled (unknown) ###
[root@rhel7u2-x64 ~]# 

• When firewalld is active or enabled, the status of ebtables is not "unknown" although its status is inactive and disabled. As a result, ebtables is output by "systemctl list-units --type service --all".

[root@rhel7u2-x64 ~]# systemctl status firewalld 
● firewalld.service - firewalld - dynamic firewall daemon 
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled) 
   Active: active (running) since Tue 2017-01-17 21:40:16 JST; 58s ago 
Main PID: 29648 (firewalld) 
   CGroup: /system.slice/firewalld.service 
           mq29648 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid                <-- firewalld is active and enabled 

Jan 17 21:40:16 rhel7u2-x64 systemd[1]: Starting firewalld - dynamic firewall daemon... 
Jan 17 21:40:16 rhel7u2-x64 systemd[1]: Started firewalld - dynamic firewall daemon. 
[root@rhel7u2-x64 ~]# systemctl status ebtables 
● ebtables.service - Ethernet Bridge Filtering tables 
   Loaded: loaded (/usr/lib/systemd/system/ebtables.service; disabled; vendor preset: disabled)   <-- ebtables is inactive and disabled 
   Active: inactive (dead)

Jan 16 12:18:18 rhel7u2-x64 systemd[1]: Stopped Ethernet Bridge Filtering tables. 
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep -e firewalld -e ebtables 
  ebtables.service                       loaded    inactive dead    Ethernet Bridge Filtering tables 
  firewalld.service                      loaded    active   running firewalld - dynamic firewall daemon 
[root@rhel7u2-x64 ~]# systemctl stop firewalld                                                    <-- firewalld is inactive and enabled
[root@rhel7u2-x64 ~]# systemctl is-active ebtables 
inactive                                                                 <-- ebtables is not "unknown" but inactive (why?). ebtables is still inactive and disabled.
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep -e firewalld -e ebtables   
  ebtables.service                       loaded    inactive dead    Ethernet Bridge Filtering tables 
  firewalld.service                      loaded    inactive dead    firewalld - dynamic firewall daemon 
[root@rhel7u2-x64 ~]# systemctl start firewalld                          
[root@rhel7u2-x64 ~]# systemctl disable firewalld                        <-- firewalld is active and disabled
Removed symlink /etc/systemd/system/basic.target.wants/firewalld.service. 
Removed symlink /etc/systemd/system/dbus-org.fedoraproject.FirewallD1.service. 
[root@rhel7u2-x64 ~]# systemctl is-active ebtables 
inactive                                                                 <-- ebtables is not "unknown" but inactive (why?). ebtables is still inactive and disabled.
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep -e firewalld -e ebtables 
  ebtables.service                       loaded    inactive dead    Ethernet Bridge Filtering tables 
  firewalld.service                      loaded    active   running firewalld - dynamic firewall daemon 
[root@rhel7u2-x64 ~]# systemctl stop firewalld 
[root@rhel7u2-x64 ~]# systemctl is-active ebtables                       <-- firewalld is inactive and disabled
unknown                                                                  <-- ebtables is "unknown" as expected.
[root@rhel7u2-x64 ~]# systemctl list-units --type service --all | grep -e firewalld -e ebtables 
[root@rhel7u2-x64 ~]#                                                    <-- both firewalld and ebtables are not output because they are inactive and disabled.

• Those behaviors are expected?
• When firewalld is active or enabled, and ebtables is stopped and disable, why does the command "systemctl is-active ebtables" output not "unknown" but "inactive"?
• When firewalld is active or enabled, and ebtables is stopped and disable, why does the command "systemctl list-units --type service --all" output the status of ebtables in the list?