Is there any guide on which fields you can join on particular advanced pdf/html templates? It seems totally random. Is there some rule? e.g. Printing invoice, why can't I join vendorbill fields? But on checks I can?

try adding &xml=t on the url of the record such as the invoice record to see base data avail. you can add additional data via script.

there is a list of transaction fields that can be pulled on advanced PDF. The list is in SuiteAnswers, try searching for it

Thanks 🙏