Proficiency is a stat earned in Returnal. As the player kills enemies each loop, Proficiency builds similar to experience. Proficiency can also be earned through other sources, such as consumables.
Each level of Proficiency earned provides a randomly-allocated stat point on any weapon Selene finds within a loop. For example, if a player is at Proficiency level 4, any weapon found will have four stat points allocated into its various stats. The player's Proficiency level only applies to upcoming weapons found in a loop and does not impact the player's current weapon or prior weapons that have been dropped.
Weapons can sometimes drop at a higher level than the player's current Proficiency level, but this is rare.
When the player dies and restarts a new Cycle, all prior earned Proficiency is lost.