Settling "What is source of funds check?" means knowing which question is being asked. Money goes in with almost no questions and comes out with several, and that asymmetry catches most people at their first withdrawal. A casino is obliged to know who it is paying, where that person lives and where the money being staked came from, and it usually asks at the moment it is about to pay rather than when the account opened. Knowing which three checks exist, and what satisfies each, turns a stalled payout into an afternoon’s errand.
Proof of address is the second, and the document that satisfies it is dated. A council letter, a bank statement or a utility bill from the last three months carries a name and an address on the same page, which is the whole point of it; a bill from last year does not. Screenshots of online banking usually pass where they show the bank's name, the account holder and the address together, and fail where they show only a balance.
The order matters less than the timing. Identity and address are often asked once and never again; source of funds arrives later, usually when the totals passing through an account cross a threshold the operator does not publish. A payout that has sat pending for days while nothing was requested is worth a question to support, because a check waiting on a document nobody asked for is the commonest cause of a stall.
An answer that will not come has consequences worth knowing in advance. An account that cannot satisfy a source of funds request is usually frozen rather than closed, with the balance held until it can, and the deposits already made are normally returnable to their origin. That is the reason to deposit from an account in the same name from the start: every check further down the line reads more easily when the paper trail begins in the right place.
Salary, pension and a house sale as ordinary answers
A frozen account, and what it means for the balance
Proof of address dated within three months
Pending payouts where nothing was requested
Source of funds asks for paper, not an explanation