A proxy in one sense is a position: a stand-in, an agent, an avatar, a functionary, and I am acquainted with the office. I have been stepson, house sitter, replacement faculty, liaison, trustee, interim director, secretary, adjunct, sub, temp, warm body, and for a short while acting editor of The Prostate–I still have the letterhead. Whose office is this? is a deputy’s question to answer, a tricky one, and also On whose behalf; on what authority do you have it?
In sciences I think proxy additionally expresses a kind of concession to imprecision, a failure. It’s the word for a subject you choose to study to produce data that can approximate the data you’d get from the actual, desired subject, if it were not prohibitively hard to apprehend.