JCS Events in Jazz Jackrabbit 2 are properties of a tile that spawn things that are neither part of the tileset nor animation data, like enemies, springs or ammo. It is currently only possible to place one event on a tile. However, belts and MCEs can be used to work around this limitation.
There are also some events that don’t appear on this list, such as the Generator event.
For the whole hierarchy, see Hierarchy of JCS Events.
Eat your lima beans, Johnny.