Warrior talent · row 2 of Arms, from level 15 (10 with Talented 5/5) · Physical

Improved Tactical Mastery

Tactical Mastery lets you retain up to an additional 3/6/9/12/15 Rage when you change stances.

Game files · build 1.60.1.70245Read from the WoW Forever beta client, build 1.60.1.70245.

Compared with Classic

Classic tooltip

You retain up to 5 of your rage points when you change stances.

Data	Classic	Forever
effect 1 aura	aura (112)	dummy (server script) (4)

What Blizzard said about Improved Tactical Mastery, each line checked against the files.

Matches the files
Tactical Mastery: Learned at level 14. Allows the Warrior to keep 10 Rage when changing Stances. Can be further improved with Arms talents.
The files Baseline passive 1310185, learned at level 14, keeps up to 10 Rage on a stance change. Improved Tactical Mastery (Arms row 2) adds up to 15 more.
Tactical Mastery Improved Tactical Mastery Class deep dives: Priest and Warrior
Matches the files
Improved Tactical Mastery: (Formerly Tactical Mastery.) Each point now increases the Rage retained when shifting stances by 3 to a maximum of 25.
The files Classic's 5-rank Tactical Mastery (5 Rage per rank) became Improved Tactical Mastery: +3/6/9/12/15 on top of the baseline 10, so 25 at 5 points (spell 12295).
Improved Tactical Mastery Tactical Mastery Class deep dives: Priest and Warrior

69876 Sep 16First beta build we archived. No changes in the client files since.