Bipeds and items
A "biped" is any character that a player can theoretically control. This includes typical Spartans and Elites, but it also includes Monitors and spawned holograms. This article is an overview of several aspects of bipeds that can be relevant when writing scripts.
Positioning
When attempting to attach objects to a biped, or position objects relative to a biped, it is important to understand the nuances of how bipeds' positions work.
A "grounded" biped — that is, one which walks on the ground, like Spartans and Elites — will have a rotation that does not match the direction that its player is looking in. Rather, the position and rotation of a biped is more closely linked to its physics and animation; typically, a biped will be facing toward whatever direction its legs are pointing in, though its facing direction can change rapidly as the biped switches between strafing and non-strafing animations.
A "grounded" biped's rotation will more accurately reflect the player's camera when the biped is in midair, but it still won't match up in real-time. If the player rapidly looks to the side while in midair, the biped's rotation will animate in that direction, possibly at a slower speed than the player's camera speed.
"Grounded" bipeds are always considered upright; even if the player is looking upward or downward, the biped's "forward" direction will always be level with flat ground.
Attachment
A biped's held items are attached to the biped; the object attachment system that Megalo has access to appears to be the same system that the game uses internally for equipped items. Similarly, a biped occupying a vehicle is attached to that vehicle.