Tags are attributes assigned to a game (like genre, setting, or other notable element). A user's top tags are those that make up a large relative percentage of their total activity (e.g. 10% of their activity is in RPG games) compared to the rest of PlayTracker.
This means that having a lot of activity in a tag does not necessarily make it a top tag if most other users also have a lot of activity in that tag.
The activity metric is an amalgamation of hours and achievements in order to account for platforms and individual games that may only feature one or the other, and does not directly represent total playtime nor total achievements.