Difference between PCTR and CCTR in SRM7.02

hello

Can anyone please tell me the difference between PCTR and CCTR.  I assume that the PCTR also behaves as a central contract if you distribute them and it will go thro PI.   Is there any restriction to have the PCTR as a central contract?  is there any differences in these two contract document types in the SRM 7.02?

Thanks in advance.

Raj