SRDF Inv Tracks

How do we interpret the difference between "0 Inv Tracks" and "- Inv Tracks" from the output of a   symrdf list ?

                              Local Device View                            

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                    STATUS     MODES                     RDF  S T A T E S  

Sym        RDF      ---------  -----  R1 Inv   R2 Inv ----------------------

Dev  RDev  Typ:G    SA RA LNK  MDATE  Tracks   Tracks Dev RDev Pair         

---- ---- --------  ---------  ----- -------  ------- --- ---- -------------

1111 1111   R1:1    RW RW RW   S..1.       0        0 RW  WD   Synchronized 

1112 1112   R1:1    RW RW RW   S..1.       -        - RW  WD   Synchronized