What is internally generated cap withdrawal?

It's an automatic withdrawal from the bank or financial institution where your account is held. It generally means that you have overdrawn your account by $50 or more (different for some banks) and the bank has allowed this to take place (for example, direct debits were honoured). It can also mean an automatic payment was denied due to the bank not allowing the account to overdraw. The fee for ANZ in Australia is $6. Other banks will differ with fees.