Secondary Skills are one of the three types of skills. OCC Skills are skills that are taught to a character or learned by the character during their process of acquiring the first level of their chosen class. OCC Related Skills are skills that related to their class, as the name suggests, that a character can select and they represent a character's choices in education during the process of acquiring the first level of their chosen class. Finally, the Secondary Skills are supposed to be skills that the character learns outside of their area of expertise and that they pick up "on the side." Unfortunately, in Rifts, most character's can only select secondary skills from their list of available OCC related skills and after first level, they gain no new secondary skills, even though they gain new OCC related skills. This does not make a lot of sense to me. In several other Palladium Books lines, such as Heroes Unlimited 2nd edition, a character gains a certain number of secondary skills that they can select from a special Secondary Skill List and that is what this document duplicates for Rifts.
In the description of each class, it lists the number of secondary skills that a character knows at first level. Use this number for first level characters, and unless a class specifies that a character gains new secondary skills at specific levels, then a character gains a new secondary skill at levels 3, 6, 9, 12, and 15. Each new skill begins at first level of experience and increases normally.