What exactly are 'one-time use' tags, and what can I use them for?
One-time-use tags are special tags that are automatically allocated each time you mark an NPC as usable only once.
When you use the NPC, all copies of that NPC definition will be removed from the map (because they all share the same tag!), and the usable only once tag will be turned on. The most common use for one-time-use tags is to turn an NPC into a treasure box.
If you want different copies of the NPCs to disappear independently, you have two choices. Either create multiple NPC definitions, which will have different one-time tags, or use the delete NPC script command instead of tags.
How else can I do this?
One-time-use tags can't be changed by textboxes. Therefore, if you are going to want to manually manipulate your one-time-use tags, you might want to use a regular tag instead.
Name a tag. Then make the NPC only appear if it is OFF. Then edit the text box which is displayed when you activate the NPC, and in the "conditionals" screen, make the tag turn ON.