This would be easier if we could see what you're working with. Just keep in mind that if you use a union query, your recordset is not going to be updatable, so you won't be able to change any values. There's no sense in making a combo box.

I can think of a circumstance where a union combo would make sense, I think.  Suppose you have 2 different tables for people and companies and you want to select from either table for a party to an agreement.  A union query would allow you to include the "Name" field from both tables and you wouldn't want to be changing it, just selecting.

That is a perfect example, but also just keep in mind that that's going to be a non-updatable record set.

It's a work project and I won't have access to my laptop until tomorrow. It's admittedly a quirky solution. We do manual adjustments to reports. We then send a confirmation email to the requester, one confirmation per email so the "paper" trail is clear.
There are three different elements we can update, and each email can have multiple requests. I started tracking this in three different tables so I can link with other info to be auto filled. I thought I could build a form with a subform for each element and send one screenshot per email linking them by the email subject line (which is a field). So this would be a read only situation.
Now I'm thinking I should just have one table for all of them, but with an ElementID field to distinguish them. Would that even help? It's a tiny database, I only got 11 emails this month. What would be a better way to handle this? And what further info should I post tomorrow when I can access the database again?
TIA

I put all three elements in one table, and it hasn't helped. Access still refuses to see the EmailSubject field when I try and link. The query I'm using for the master form has unique values, is that causing an issue? Below is a visual of what I'm trying to create, either with a dropdown in the master form or by clicking thru the records on the master form.

I cracked the code on why so much of Access didn't want to acknowledge the EmailSubject field! I forgot I created it as LONG text instead of short text. 🙄