For weapon skills, it only changes when players change their equipped weapon; in which case all 5 slot skills will change, except in the case of changing an offhand weapon. [Guild Wars 2 main page]
For slot skills, each one can be changed out individually depending on which ones have been bought with skill points. The first slot skill, skill slot is reserved for healing abilities, which players usually don’t have many of. The next three are the normal skills, and they have quite a few options for players to pick from. The last slot is reserved for elite skills; players only have around three of these types of skills.
Unlike most other MMORPG’s, Guild Wars 2 has a very unique skill slot system. This system doesn’t allow for quite as much customization as some other games, but at the same time it offers the most unique ability customization out of any MMORPG out there, in its own unique way.
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