What is __POCC_STD_VERSION__?

[ranadas]

What is the meaning of the following piece of screenshot found in "crtdef.h" in Pelles C Include Files.

Below is the attachment!!

Further what is the meaning on __POCC__ >=600 && !defined(__midl) ?

Please Help I'm an absolute beginner!

Also what is _CRTCHK(f,n,m)  function?