What is the meaning of "show paths" output in 'multipathd -k'

Issue

• Run multipathd -k, and execute show paths command, for example:

multipathd> show paths
hcil    dev dev_t  pri dm_st   chk_st  next_check      
0:0:0:1 sda 8:0    1   [active][ready] XXXXXXXX.. 16/20
1:0:0:1 sdb 8:16   1   [active][ready] XXXXXXXX.. 16/20
2:0:0:1 sdc 8:32   1   [active][ready] XXXXXXXX.. 16/20
3:0:0:1 sdd 8:48   1   [active][ready] XXXXXXXX.. 16/20
4:0:0:1 sde 8:64   1   [active][ready] XXXXXXXX.. 16/20
5:0:0:1 sdf 8:80   1   [active][ready] XXXXXXXX.. 16/20
7:0:0:1 sdg 8:96   1   [active][ready] XXXXXXXX.. 16/20
6:0:0:1 sdh 8:112  1   [active][ready] XXXXXXXX.. 16/20
#:#:#:# vda 252:0  1   [undef] [ready] [orphan]        
#:#:#:# vdb 252:16 1   [undef] [ready] [orphan]        

• Need to know what are the meanings of undef and orphan, and the meaning of next_check field