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Only the skills marked in bold font are relevant to the performance of the NPC in question (their Primary Skills). In most cases, each NPC will have three Primary Skills, but Engineers and some Specialists only have one. Marine Officers are unique in that they have a skill that improves with use: "Boarding Experience". For all NPC types where we know the relative significance of their primary skills (see below), they have been listed in descending order of significance in-game. So for those NPC that we don't know the relative significances of their primary skills, it is reasonable to assume that they may be in descending order, or of at least equal weighting.
In Rebirth 3.00, Virtual Seminars were introduced to the game, which allow us to add a star to a Primary Skill of our hired NPC. They are only currently available via Smalltalk from other NPCs and this feature is described in more detail in the official guide to 3.00 here and on this wiki here <edit: they may be obtainable as mission rewards too; needs checking - Snafu>. Virtual Seminars cannot be used to increase a Marine Officer's Boarding Experience, but since Virtual Seminars take some effort to acquire I would suggest that would be a waste anyway.
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