Parkour
Marines have several movement options that normal monsters do not possess. This page will cover each of them. They can also all be disabled with the User_NoParkour user variable. And individual moves can be disabled with the Action restrictor.
Marines can climb over blocking actors, 3D floors, sectors, and 3D middle textures. Allowing them to get over obstacles that'd stop other NPCs. They can only climb to ledges that are 98 map units tall or shorter. If the ledge is 61.6 map units tall or shorter, marines can climb it as-is. If it's taller than that however, they will first jump up to reach it, and grab it, THEN climb. Also, if -fastmonsters is enabled, they will be able to climb up twice as fast, while if SlowMonsters is enabled, they'll climb up only half as fast.
In addition to being able to climb up, marines can also climb down ledges shorter than 98 map units by jumping down from them, since they're able to climb back up those ledges again.
Marines can jump over gaps in the map, be they made out of level geometry, 3D floors, or actors. If the gap is 192 map units wide or less, marines will just do a normal jump. If it's longer than that, marines will only be able to jump across if they can walk back 256 map units, then sprint forward to do a running jump, if anything blocks their way during that process, they will not be able to do the long jump. Marines can only jump over gaps as wide as 288 map units or less.
Marines can crouch under 3D floors, actors, and normal sector planes that have a height less than 56 map units, but that are at, or over, 30 map units. Allowing them to walk under tight spaces like players can.