Rigify is seems to be a popular plugin for rigging. If a person is using human rigging based on the Rigify, are there any pre-defined poses or library for posing?
As Rigify is a very flexible tool, a pose that works for a Rigify Character will not work with a different Rigify Character. To transfer poses (and animation) from a rig to a similar one you should do a "retargeting", which is an option that some softwares have (like UE4), but not Blender, unless there is some addon that I don't know.
if rigify were a very flexible tool as you say,a pose that works for a rigify character will work with a different rigify character ! yeah,you can use the retargeting but it takes a lot of time.