A frame of reference in which poses may be expressed. Name of the frame. It must be unique within its scope (model/world), i.e., it must not match the name of another frame, link, joint, or model within the same scope. If specified, this frame is attached to the specified frame. The specified frame must be within the same scope and may be defined implicitly, i.e., the name of any //frame, //model, //joint, or //link within the same scope may be used. If missing, this frame is attached to the containing scope's frame. Within a //world scope this is the implicit world frame, and within a //model scope this is the implicit model frame. A frame moves jointly with the frame it is @attached_to. This is different from //pose/@relative_to. @attached_to defines how the frame is attached to a //link, //model, or //world frame, while //pose/@relative_to defines how the frame's pose is represented numerically. As a result, following the chain of @attached_to attributes must always lead to a //link, //model, //world, or //joint (implicitly attached_to its child //link).