Design Pattern: Ability Slot

An ability slot is similar to an upgrade of choice, but the choices available do not represent a permanent change to a game entity's abilities.  Instead, the entity has a certain number of slots which abilities 'take up' when equipped.

I cannot think of a real example where this would be used and not be pigeon-holed, but that does not mean it can't and hasn't been done.  Certainly, one could have a game where the player's character has three slots and only three abilities to fill them with, but with the option of putting an ability in multiple slots, thus augmenting that particular ability at the cost of flexibility.

An important feature of ability slots is that shuffling abilities around is in some way a hassle.  For instance, you may not be able to do so during combat or outside of a certain location.

Examples

D&D

A magic-user in D&D has a certain number of spells they can cast per day, unless you're a heathen and play 4.0 where you have a certain number of spells per battle, because who wants to deal with all of that pesky role playing business anyway.*

You of course need to learn the spells first, but after that, you can prepare any spells you have for the day.

FPS's

FPS's basically always have ability slots in the form of weapon choice; while gaining access to the weapon is usually an upgrade of course, which weapons you have equipped is almost always an ability slot.  Many games have dual-wielding now, and Skyrim of course makes double-equipping a spell a whole lot of fun.

Raiden

I'll be honest and say I played a fan homage Flash version of Raiden on the internet, but the important thing here is the fact that you have one ability slot, your weapon type, and three types of weapon.  Raiden also exhibits the design pattern upgrade track.

*Seriously, do not play 4.0.  Having spells per battle instead of spells per day messes with the tenuous sense of scarcity in a game where magic users were already incredibly powerful after a certain level.  The skill Intimidate can't be used outside of combat; I suppose it is better to simply kill all your enemies than to save one to interrogate later.