You can use any playerskill with the skill display templates, and some even take monster skills. There are a few special generic icons that may appear on a Build or Skill bar listed on this wiki or other sites.
Empty
Empty[]
An Empty slot has no skill in it, and is supposed to be that way. Many builds that are predetermined by the game, such as Norn Blessings, have empty slots in them.
Optional
Optional[]
Optional slots are usually used in Player-created builds to denote a skill slot where any of a variety of skills could be placed with equal effectiveness.
A player-created build that uses no elite might use this icon to show that a slot for an elite skill is still available.
Templates[]
You can use these templates to display skills and icons. Some do not work with monster skills such as Knock. The template pages available through the links in the right column often have more thorough documentation that may indicate limitations and additional options. See Category:Templates/Skill display.