Got a question from a reader of this site asking, "So what exactly is a TV tag?" Guess I should have explained that.
When you produce a TV show for a network, there are specifications. The show is supposed to be delivered at a specific length and the opening titles and closing credits are to be a certain length. If there are commercials involved, producers are told how much time to leave for commercial breaks and approximately where in the show they are to fall.
If we're talking about a half-hour show as we were with M*A*S*H, the specs usually lead to the decision for a two-act structure. The show is to be told in two acts…once in a while, three. But these acts may be preceded by a very short scene at the top to try and hook the viewer…or it may be followed at the end by a very short scene that provides a feeling of "wrap-up" and if it's a comedy, ends on a joke. Once in a while, you do all of these.
If the scene at the top starts your show and precedes the opening titles, it's called a cold opening. If it comes after the opening titles, it's called a tease or teaser. The scene at the end is called a tag. These are not set-in-titanium terms. They're just what most people call them.
When I worked on Welcome Back, Kotter back in the ice age, this was the format: The first thing you saw was a cold opening in which Mr. Kotter told his wife a joke. This led into the opening titles, which led into Commercial Break #1. Then we had Act One, followed by Commercial Break #2, then Act Two, then Commercial Break #3. When we came back from Commercial Break #3, we had a tag in which Mr. Kotter told his wife or someone else a joke and this led into the closing credits.
Generally speaking, you want to make the tag expendable because when they start cutting the show for more commercial time in syndication, that's usually the first thing to go. Why have it in the first place? Because it may cause some viewers to sit through the last commercial break to see it instead of turning off the set, changing channels or heading for the bathroom. So advertisers like tags and so do networks. That last commercial break is where the network will probably put a promo for the show that's on next, the goal being to stop viewers from reaching for the remote. And that's about it.