When is BTO income ceiling calculated? During BTO application or during HLE application?

Should be at the point of application - you can check the eligibility criteria - https://services2.hdb.gov.sg/webapp/BP13EligChe... - based on last 3 mths income before application date.

I believe upon application - all income documents have to be formally submitted - this is where they confirm whether one is eligible.

Application shld be before balloting.
HLE is mainly for loan application / financing the HDB flat.

I think if one is just thinking - haven't put in application yet (1st initial step) it's not deemed as serious enough so the info appears to look vague (available one HDB website if look hard enuf) - the information becomes very clear when one is in the process itself (i.e. use SingPass and start the application).