This read-only Unsigned32 gives the bytes remaining in the outbound LLDPDU after all TLV groups have been added; if overloading has occurred, it instead reflects the additional space needed for all groups combined. It's the aggregate counterpart to the per-group left-size field. For example, seeing the shortfall reported here after adding both the location and MED capability groups together tells the admin how many more bytes would need trimming to fit everything into one LLDPDU.