For example, suppose that you selected Engineering as your initial branch at Starfleet Academy. As an Ensign, you go do some missions and earn some additional points to spend on new skills. When you return to your home Starbase, you get a choice of skills to learn:
|2 pts||Warp Field Analysis II|
|3 pts||Power Generation III|
|1 pt||Replicator Systems I|
|1 pt||Transporter Systems I|
|2 pts||Sensor Enhancement II|
|2 pts||Scanner Operation I|
|4 pts||Tricorder Operation II|
|4 pts||Botany II|
|8 pts||Genetic Assay III|
|2 pts||Astrometrics I|
|2 pts||Direct Fire Weapon Targeting I|
|4 pts||Seeking Weapon Targeting II|
|8 pts||Seeking Weapon Programming III|
|2 pts||Communications Systems I|
|4 pts||Personal Energy Weapons II|
As this sort of thing gets repeated over time, most players will wind up choosing more skills within their initially selection branch because they "get more for their money" by doing so. But they can still take a few skills from outside their branch to help give their character a unique identity.
Incidentally, this is a good example of how an economic approach to resource allocation (let players "pay" for in-game rewards) can let players make interesting choices while still promoting certain outcomes that are desirable from a gameplay balance POV. In general, if there's something you want people to do, it's better to give them a reason to want to do it than to force them to do it by constraining their choices completely.
Sometimes it's necessary to put players on rails, but for RPGs it's more fun when you leave them free to do interesting things.
Of course, that doesn't mean you can't impose consequences for the actions they freely choose to take. :)