Working out "What is source of funds?" 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 a short errand.
Source of funds is the third and the one people find intrusive. It asks where the money being gambled came from, and the answer is a document rather than a sentence: payslips, a tax return, a contract of sale, a statement showing the deposit arriving from a named employer. A salary is a perfectly ordinary answer and so is a pension, a redundancy payment or a house sale. What fails is an answer with no paper behind it, because plausibility is not what the check is asking about.
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 a balance is all they show.
The identity check is the simplest. A government document with a photograph, a full name and a date of birth settles it: a passport, a driving licence or a national identity card. The name on it has to match the name on the account exactly, which is where most rejections come from — a shortened first name, a maiden name, a middle name present in one place and absent in the other. Photographs of a document are read by a person or a machine, so the corners have to be in frame and the text legible rather than washed out by a flash.
Source of funds asks for paper, not an explanation
Pending payouts where nothing was requested
Salary, pension and a house sale as ordinary answers
When each check arrives, and why the third comes late