In an RPG that uses Character Classes, there can be many different ways to do it. There's the many variations on the classic Job System, there's the Character Class System, the Class and Level System... but the most popular way of doing it is probably the Skill Slot System.
Simply put, a character, when choosing a class or a job, has a choice of several skills, and at a certain level the character learns a new command/innate ability. When the character reaches said level, they'll gain the skill, and can assign the skill to one of (usually four) slots, and they can then use it. Unlike systems like the Tech Tree which require points to be invested in skills in order to make them stronger, a Skill Slot system's skills usually get stronger automatically as you level.
One of the big draws of the system is that changing up on skills is easy to do, allowing you to strategically choose skills for the best advantage in any given situation. There's usually a skill menu in game that will allow you to do it, though access to it varies with each game. Some games may allow you to change skills on the fly, but for others, you may only be able to change skills between stages or missions, making choosing the right skill very important for success.
- Final Fantasy V introduced the Job System. Most jobs have four ability slots. For many jobs, two of them are taken up by Attack and Items. One ability slot is taken by whatever ability the job specializes in (weapon techs for fighters, spells for mages, and so on), but the fourth ability slot can be used to access another job's abilities, such as a Fighter using a White Mage's healing abilities.
- In Granblue Fantasy, the player can select a subskill for any of the captain's classes (one for Tier 1-3 and EX 1 classes and 2 subskills for Tier 4 and EX 2 classes), and/or one Extended Mastery Skill depending on the class line.
- In Blue Dragon, you fight by means of using Shadows, each of which can choose between Jobs. All Shadows have an ability that cannot be changed out, but other than that, any ability a Shadow has unlocked in any of its Jobs can be slotted and used freely.
- Diablo III allows two mouse-button skills and four other active skills to be slotted, in addition to up to three passive skills, a very welcome change from the skill-tree system used by its predecessor, Diablo II.
- One Finger Death Punch allows you to learn skills that you can then slot to kick more ass. Up to three skills can be slotted at any one time, and all skills are passive, activating when you kill a certain amount of bad guys.
- Heart of the Swarm: Kerrigan has several skills, both active combat abilities like mass-levitating enemies or summoning reinforcements and passive ones that increase her strength or attack multiple enemies. She also has skills that are more useful for running the base, like twinspawning drones, faster-spreading creep or instaspawning overlords. While these can all be switched around before the mission, the game needs to be beaten before you can choose which unit variant you want on a mission.
- Legacy Of The Void uses a similar system, except all skills require a different amount of solarite, so some juggling is required to get the useful skills.
- Sonny lets you choose which skills you give your character, with some skills being available several times (on separate cooldowns).
- Superdimension Neptune VS Sega Hard Girls gives each party member three skill slots to start out with. You can bump it up to five if you find and complete the associated quests.
- In Armory & Machine, you start out with one skill slot but gain more from defeating bosses, up to six. You can then fill them in with skills from one of the three skill trees (Soldier, Inventor, Hunter) as you unlock them.
- Armory & Machine 2 continues this like the previous game. You start out with one skill slot, but can research up to eight (although the seventh and eighth slots take a long while). You can then fill them in with the very impressive skill arsenal you obtain.