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?

we are all lost on this... over time you get better and learn from these boards. xml=t helps, record browser helps, entity vs transaction records have different issues. there are hacks around it all if you program or are willing to learn some basis things. in general NS has hidden items, some joined items u don't know, and some non joined items u expect. ther eis no easy button I have found other than time and trial