Read the Regs
CAO 82 lays out the rules. basically a CP has to be current to execute his duties such as Training and Checking etc. To do this he has to be current, hold a valid medical, and a current MECIR if the organisation does IFR charter. In addition he has to be endorsed on all types operated by the company although I understand this may not always be the case especially in large companies like QANTAS. I am sure someone will correct me but this is a general outline of the way I see it.
Groggy