Anything they want.
For example the button monkey in the branch does your application and sends it to underwriting. It's queued until someone looks at it.
If they want to know about where a lodgement came from an email gets sent to button monkey in branch, they contact you and you explain lodgement. Button monkey sends reply to underwriting. It joins the queue again until someone looks at reply. Then they might want to see an extra document to support what you said or for something else. Email sent to button monkey again etc etc and just repeat and hope that you don't lose your mind during the process.
Their underwriting dept. Lost my application for weeks. The button monkey didn't send up all the paperwork and then took holidays. Button monkey 2 took over. Branch manager got involved. 3 months later they admitted they fucked up and then declined the application. I have no idea if the underwriter ever got tge full application info.
I have given 12 months savings and current account statements. As well as 12 months mortgage statement. P60. Salary Certificate. Can they ask for more than 12 months statements.