Just wondering, I've been cleaning mods with TES tool. Also what does it mean by "updated cells and removed items"?

And not related to modding but when you install, what purpose does registering the game online serve?

It improves performance in that if there are conflicts, or if there is something removed that another mod acts upon, the game won't crash.

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Intellectually it should improve performance. By removing records from a plugin there are less records for the game to process on loading and when playing.
Whether you'd actually notice a difference, IDK.
Cleaning by TESTool or (better) tes3cmd means that records which're identical to those in the plugin's masters are removed.

If the mod with dirty records loads after one which intentionally changes those records then the intentional changes are lost.
Have a look at the TESTool readme.
Spam?