Compartmentalization perk is one of four perks that essentially provide a free mastery point:

It makes Lightweight Equipment Mastery free to take immediately upon the start of the game, without spending a mastery point. Compartmentalization is a unique perk in the sense that it does not automatically pick Lightweight Equipment Mastery; it just removes its Mastery Point cost. It is up to the player to pick Lightweight Equipment mastery, for free, during the game.

It still costs 7500 minerals to fully complete. If a player wants to take both Lightweight Equipment Mastery and Weapons Research Mastery at level 0, and spends a perk to do so, the efficient way is to take Weaponsmith over Compartmentalization because Weaponsmith provides a fully researched mastery, whereas Compartmentalization still requires research time and resources. If a player takes Compartmentalization and takes both Weapons Research and Lightweight Equipment Masteries at the start of the game, they will be at a significant disadvantage while both fight for limited resources.

Compartmentalization Perk has also been covered in the official Perk Detail Overview on BlacKcuD's blog.

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