All these information is available in UCCE SRND, as well as admin guides. you must start exploring them.
let me also try to explain in brief:
PG connects to peripheral like CVP, CUCM etc. the pim process is the one which actually makes a connection to external peripheral.
a single PG can connect to multiple peripheral using multiple pims(a PG can have multiple PIM's), and each pim is configured in ICM as a separate peripheral, and for the same pim you will also have a separate routing client.
e.g A VRU PG has a 4 pim connecting to 4 CVPs, then basically you will have 4 routing clients. each as a separate peripheral.
regards
Chintan
