Halo Custom Edition can let you play custom content, which greatly lifts the limits of the original Halo. HCE also has an editing kit as a separate application.
The engine has limit capability, which has become a serious stress for most mappers. The Custom Mapping Team is one of these mappers, and the engine has become a great hindrance. One of the greatest concerns is tagspace, which limits the amount of tags imported into Sapien. This, however, is not a concern for most mappers.
Most mapper have bypassed the engine limits, such as allowing dual-wielding and vehicle boosting, although perfectly imperfect. Dual-wielding classifies the two weapons as one, and replaces the grenade trigger with the secondary weapon trigger. Boosting puts a "fake" damage indicator, usually behind the player, which is set to deal 0 damage. Boosting, however, often screws up the vehicle, and takes it random places, unless the damage indicator is in the right place.