Hi!
Each secondary skill lists "Related Skill Categories" in the chart on page 35.
When you advance a level, you can use development points from those related categories to advance your secondary skills.
See note, page 246, chart CGT-4, at the bottom.
Example: Your Scout character desires to improve their Gambling secondary skill upon gaining a new level.
General and Subterfuge are the related categories for the Gambling Skill.
Your Scout may use development points from General (3 points available) or Subterfuge (5 points available) to increase their Gambling rank.
(see page 246, Table CGT-4 for development points gained)
Hope this is some help, have fun!