Placement can be thought as an additional transformation of the tracking data that depends on the Alt's position relative to the pivot point of the object the Alt is affixed.
Any virtual object has a pivot point that sets the position and rotation of this object. For example, for an head-mounted display (HMD) this point is located between the user's eyes.
When you setup the Alt on a physical object, the Alt's pivot point may not coincide with the object's pivot point in virtual reality.
Therefore, in this case, you have to determine the displacement and adjust the Alt's pivot point relative to the position of the point between the user's eyes.
The pivot point of a gun controller can be located near the handle. In this case, the Placement transforms the tracking data in accordance with the offset and rotation of Alt relative to the pivot point on the weapon handle.
The pivot point of the Alt is located 2.62 mm from its backside.