The Ladder actor is a placeable navigation point marking the bottom and the top of a LadderVolume so the AI knows how to use the ladder. The AutoLadder is the automatically-placed version for LadderVolumes with bAutoPath set to false. Usually you can let the editor place AutoLadders. Only if you are not happy with the automatically placed ladder markers you should place the Ladder actors manually.
See Ladder (UT) for the Unreal Tournament class describing a single player ladder.
- Add A Ladder – how to add ladders to a map