How to set or change the default soft or hard limit for the number of user's processes?

Issue

• The system wide configuration file /etc/security/limits.d/90-nproc.conf (RHEL5, RHEL6), /etc/security/limits.d/20-nproc.conf (RHEL7) specifies the default nproc limits as:

  *            soft    nproc     1024
  root       soft    nproc     unlimited
  

• However, when logged in as root, the ulimit shows a different value:

  $ ulimit -u
  1024
  

• Why it is not unlimited in this case?

• How to set or change the default soft or hard limit for the number of user's processes?