I don't think there is any need for a specific document. Their website, now a days, specifically mention not to include any property valuation documents, income sources and bank statements.

But these could be the starting point:

1) A general one page summary (MS Word, Word Pad, Open Text) why the person is wishing to visit.
2) Letter of Invitation. If this is one pager (MS Word, Word Pad, Open Text) is written by a family member then perhaps point no. 1 is not needed. Letter of invitation from institutions or conferences, I meant.
3) An ID of the company or institution that the visitor is affiliated with. I don't think a letter from that employer is required here (if the visitor is working in Nepal).

And that's about it. They (the Counselors) pretty much guess, mind read, observe, back-end check and decide on the spot.