zu 1
zu 2

Erläuterungen zu vorangegangenen Stichpunkten.
Zumindest wäre das der korrekte Gebrauch

This document is to be translated into English, I thought maybe Ad = addendum, but I'm not sure.

not quite sure what you are getting at... you need an english translation for Ad) 1, which you found in a German text?!
Addendum is more the heading of the chapter, similar to Appendix (Anhang), followed by a numbered or bulleted list.
If you want Ad 1, Ad 2, etc. which refers to notes earlier on (like footnotes) i'd go for 'Re:' (regarding, referring to)
if this is not what you're after, please elaborate further.

I believe it is the Latin "ad" (zu) and is used in English as well.

However it seems not to be well known in America and among the younger generation.

Probably better known in the legal profession.