BUR #16918 has been approved by @nonamethanks.
create alias behind_back -> holding_behind_back
create alias object_behind_back -> holding_behind_back
The point of both tags are similar according to behind back's Wiki. Meanwhile posts with object behind back shows characters holding an object behind their backs. I will welcome other options or changing this BUR when most of users would disagree with this.
While *_on_back tags refer to objects that are not being held by the character but rests on their back.
You can edit my forum post if there is something wrong.
Updated