You can check here the bridge process.
In resume:
BILLING
Charge Out - The cost is reflected in destination type Billing. If it is a Interco billing the info of partner responsible CC and GBU
Charge In - The cost perspective of the Billing receiver GBU
INTERNAL ALLOCATION
Charge Out - The cost is reflected in destination type PL. The cost moves from a GBU at Origin to another GBU in Destination

Charge In - The cost perspective of the Internal Allocation at the receiver GBU


