Scenes used to be in
xml (XML) and
scn (binary) formats, one that is human-readable and another for performance.
tscn (TOML) was added to the engine as a more friendly text format, and
xml is now deprecated.
Resources that are not scenes also use these same formats, under a different extension suffixed
*res so they can be differenciated from scenes.
The idea is you can use
tscn (with a
t) to save resources and scenes as text format that is easy to work with during development of the game (especially with Git or other VCSs), and then use
scn if performance is really needed on a huge scene that takes forever to load for example (they are converted to binary anyways when you export the game).