I am assuming the tablet will be IMAP. If so, a trash folder on IMAP is not accessible from POP account. I suggest creating a folder on the IMAP acccount, moving important messages there and, on returning home, move those messages back to inbox prior to using the POP account.

"I am assuming the tablet will be IMAP". Tablets are neither inherently IMAP nor POP3. The Android version of Thunderbird supports both protocols and appears to also otherwise have the same options as the PC version. So back to my question:

When I set up a POP3 account on Thunderbird (regardless of where I'm running Thunderbird), which specific action constitutes a "deletion" of an email?

Deleting from trash deletes from server on next access to server. My earlier comment was that saving to trash on the phone causes it to not be accessible from a POP account on PC.

"saving to trash on the phone causes it to not be accessible from a POP account on PC"

Now I am totally confused. If putting an email in "Trash" does not delete it from the server, then why would my PC version of Thunderbird be unable to access it? It seems to me that a POP3 email is either on the server or it isn't. If it's still on the server, then any of my clients should be able to access it.

When you delete an email from your inbox Thunderbird will delete that same email on email server the next time that Thunderbird execute action "Get email" ( Pop3 is not instantaneous like IMAP ; Parameter Check for new messages every N minutes ) or when you explicitly request an "Get email" action. Thunderbird will not "wait" until you delete that email from trash folder.

Example : Parameter Check for new messages every N minutes is set to 30 minutes. You delete email "A" from Thunderbird at 10:00 AM . Email "A" will still exist on the email server until 10:30 AM at most ( 30 minutes later ) or until the next "Get email" is triggered manually ; so between 10:00AM and 10:30 AM another Thunderbird client may still able to that email downloaded to it ; if a manual request "Get email" is triggered at 10:15 AM then email "A" will be deleted at 10:15 AM on the email server. All this is in the context of the Thunderbird Client that trigger the delete action. However Email "A" may still exist in your Trash folder of Thunderbird client and Trash folder of email server.

Well that is actually the answer to my question, so thanks. That's unfortunate, but I just tested this after setting up my Android client and you seem to be correct; when I removed a piece of email from my Android Thunderbird client Inbox and synced with the server, the Android client deleted it from the server. That is unfortunate, because it means that I cannot use "Trash" as a temporary repository for items that I want to keep while also using it to delete the messages I don't want to keep. It seems much more rational to me to regard deletion from Trash as the trigger for a server deletion, rather than removal from Inbox.

I'm looking at the Android version and I don't see a way to create a set of user folders. I was hoping to create a user folder for temporary storage of desired emails that would not be deleted from the server, thereby allowing me to clear my Inbox for uncluttered viewing of new incoming emails. My thought was that I could move emails from my Inbox to this temporary folder without deleting them from the server, while eliminating unwanted emails from the server by putting them in Trash. But since there seems to be no way to create a user folder, this option seems unavailable.

Generally it seems to me that POP3 email clients could very easily provide specific options for exactly when emails should be deleted from the server. But it seems that the makers of almost all clients feel that users have no need of anything other than a vague and general notion that emails do or don't get upon fetching of the email or upon removal from the Inbox; there seems to be no conception that users might be using multiple instances of their clients in situations like mine.

You're now back to my suggestion of creating another folder. If you can't do on Android, you could create the folder via your online account. Then, move to that folder from the phone. Then, when you want the mail viewable from PC, move back to inbox with the phone. Moving to Trash on the phone wouldn't work anyway, as trash on the phone is not the same trash on a POP pc account.