A classless character system replaces class-based specialization:
Typical approaches involves artificial or dynamic skillsets designed or chosen by player. Player can still be limited by their faction/race instead of class, making some skills unavailable.

1.All skills are available, but only N skills can be learned by a character creating a limited skillset.
2.Skills can be chosen, but only one from each group of skills.
3.Specialization allows to choose a group of skills, excluding other groups in a skill tree.
4.Amount of skillpoints is fixed that prevents using most skills beyond first few levels, forcing to invest most points in 1-2 key skills.
5.More skills are unlocked at higher levels, limiting spending at lower levels. Most RPGs with free skill systems use this.
6.Skill tree requirements require learning a sequence of skills, each of them requiring a specific level of the previous skills, limiting skill points use to only a few branches.