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.
Compare Job System, Class and Level System, and Character Class System.
Examples:
- In the Armory & Machine series:
- 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.
- 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.
- Though not an RPG, Civilization VI harkens along its iteration's theme of crafting a story for a particular civilization's history by introducing a civic card slot system where players can choose among different civics cards that offer different bonuses, such as the early-game Discipline card that grants units +5 strength against barbarians. The slots are divided into military, economic, diplomatic, and wild-card types, and as the game progresses new research increases the variety and strength of those cards. Adopting new government types is the usual way of expanding the total number of slots available, though some civilizations like Greece or Germany get a bonus slot and a few World Wonders grant the owner bonus slots as well. Cards can normally be swapped in and out on the turn a civic technology is finished, though gold can be spent to change them outside of those turns if necessary.
- 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.
- Divinity: Original Sin II: The Memory attribute determines how many skills a character has access to in combat. Each skill requires from zero (i.e., always available) to three Memory slots; outside of combat, players can freely decide which of their known skills are in their Memory. The attribute scales with level and can be boosted with Points and some items.
- In Granblue Fantasy, the player can select a subskill for any of the captain's classes (one for Tier 1-3 and EX1 classes and 2 subskills for Tier 4 and EX2 classes), and/or one Extended Mastery Skill depending on the class line.
- 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.
- 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.
- Pillars of Dust: Carlton and Gregg can only equip four skills at a time, but can reassign skills outside of battle.
- Re:Kuroi: Characters can equip five skills, but each slot only allows certain elements and the fifth slot can only be used for common skills.
- Soma Union: Unlike the previous game, Spirit abilities are equipable, but there's only one of each ability, meaning the player has to figure out which ability works best with each character.
- Sonny lets you choose which skills you give your character, with some skills being available several times (on separate cooldowns).
- StarCraft II:
- 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.
- 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.
