You choose two careers at the beginning. You start in Tier 1, after taking two Tier 1 skills you unlock the skills in Tier 2 and so forth.
Having 2 skills from Tier 5, unlocks a new career.
Tier 1 skills cost 200xp, Tier 2 cost 300xp, Tier 3 You can dual wield pistols and small firearms, without any penalties cost 400xp, Tier 4 cost 500xp, Tier 5 cost 600xp.

Red Careers are mainly licenced, here you don't take skills with xp, but spend time and credits to gain new skills. Tier 1 is one week and the other Tiers
have the number of weeks required listed on them.