Start pos is an events category in JCS, consisting of three events that manipulate where in the level the players spawn. In Single Player mode, none of these events is absolutely compulsory (if no start pos is found the player will fall into the level between coordinates -5,1 and -6,1), but multiplayer games cannot be run without a start pos event. Placing a start pos event right of a masked tile will cause the player to be stuck in it. This applies to Warp events as well.
This is a subcategory in JCS.
Eat your lima beans, Johnny.