It's a while since I worked on TSX but I think this is an offset so if the value of %MW11 is 0 then the value of %M130 is used . If % MW11 is 1 then the value of %M131 is used and so on . Other contributers will correct me if I'm wrong .
Paul
it is indirect addressing.
It should not be less than 0, it will cause inder error warning on program side.
(it was system bit 16 or 18 maybe and you can select with another system bit if CPU is halted or stays at run)