To achieve the desired hierarchy where the R_arm layer is nested under a R_arm bone within the torso bone, you can use the [bone] tag to create a bone for R_arm and then manually adjust the hierarchy in Spine after import. Unfortunately, the current tagging system does not support directly specifying nested bone hierarchies through tags alone. You can tag R_arm with [bone] to ensure it creates a bone, and then after importing into Spine, you can manually move the R_arm bone under the torso bone in the hierarchy.
Here's how you can tag your layers:
R_arm [merge] [bone]
torso [bone]
