Bone copy is a script for 3ds Max, made available by developer Chuggnut. Its function is to swap the skinning weights from one bone in a Skin modifier to another, transferring the weighting from a chosen source bone across to a second bone.
Skin weights in 3ds Max control how each bone pulls on the mesh assigned to it. Bone copy focuses on moving those weight values from a source bone to a destination bone inside the Skin modifier.
The current release is version 2.3.5, and it is listed here as free. Bone copy appears in the ScriptSpot library, and Chuggnut's own page at chuggnut.com is the spot to confirm the latest version and any specifics on how to run it.