Can this part be used to put a bike seat on a bike seat post or is it just for putting a bike seat post on to the frame?
That part is to put a thinner seat post in a frame with a wider seat tube. I really don’t understand that bit about ”put a bike seat (AKA saddle) on a bike seat post”. More basic seat posts usually use this kind of clamp:https://www.amazon.com/Action-Saddle-Clamp-Standard-Single/dp/B001CJCT02/ref=mp_s_a_1_3?ie=UTF8&qid=1547540011&sr=8-3π=AC_SX236_SY340_QL65&keywords=bicycle+saddle+clamp&dpPl=1&dpID=41sa8DNrJzL&ref=plSrch and I’ve only ever seen them in one size. (Post diameter may vary, but clamp area diameter I’ve only seen one) Better posts generally have an integrated saddle clamp that isn’t replaceable.
You can use a bike seat post shim to put a smaller diameter seat post into a larger seat post tube. However common sense should tell you that you can't put a larger seat post into a smaller seat post tube. Without knowing the particular saddle you're talking about using; it's a guessing game as to what might work. Therefore edit the question and give specific information about the saddle. Or take the saddle and seat post to a reputable bicycle shop so they can suggest the best options for you. If you have a smaller seat post the saddle will fit on then consider getting a shim for the seat post tube. If the seat post tube is too large. Your question is confusing. It sounds like you're also saying the saddle clamp won't squeeze in tight enough because the seat post is too small. Meaning you can't use a larger seat post because the seat post tube is to small. I personally wouldn't trust shimming the seat clamp to the seat post. Here I'd advise welding the shim onto the top of the seat post.