MS > OS Meaning

So TLdr; break down what MS > OS is. / Pugging into dps roll but your talent spec is mainly tank because you tank, what is your "MS" thenI got into a pug as a gnome warrior. I'm always fury prot and help OT in every occasion I can, even to save the raid when other warriors are afraid to step in. I showed up as a DPS role with intentions of rolling on tanking gear. Some t3 dropped and i rolled to which i was a high rolling winner, to which I was told im a dps warrior thats not my MS. I let them know i was fury prot and could confirm my spec if needed. My question is... was I in the wrong to roll on the gear? Should I have specified my MS loot priorities was tank gear? If i was in the wrong and shouldve been only rolling on dps gear should I have respecd or stayd fury prot.