What makes a player a WR,RB?

[onlooker]

D.McCluster always seems to get the tag of WR,RB. Why doesn't D.Sproles or P.Harvin? Do they have to line-up at a position a certain amount of times to get the added position tag? D.Sproles and P.Harvin are all over the field, but don't get the multiple position tag.

[dudekid]

i agree, i find it pretty strange that ONLY mccluster seems to ever have this tag...i think a year or 2 ago he was actually the RB for the chiefs, but his primary position is WR...still not sure why harvin, cobb, aaron hernandez, etc, never get this tag as they all line up and take handoffs

[Dux90]

I also think that Harvin should have it he does it all why not give it to him??