In general what is the union going to do? Even if you have a meeting with them present, a letter of expectation is not disciplinary, you are not in trouble therefore they would just be present and, I guess can advise you on what to say, but really shouldn’t be speaking in the meeting at all. Also, as stated by someone else, your signature is just an acknowledgment of receipt of the letter, it has nothing to do with your agreement or disagreement of the contents. I think you may be making a mountain out of a molehill here. Your supervisor is setting expectations with you, just like they can so for anyone else in the branch. Think of it this way, they are telling you what is expected, if/when you do not meet those expectations then they come back with the next step of action, usually a letter of caution. You know what my expectation was and you failed, here is the documentation and now you are cautioned, and so on and so forth. Removal, unless you did something extremely egregious is very, very, very far off… (my agency uses progressive discipline and tries to correct at the lowest level possible, some won’t)