It could mean lots of things, but more than likely there's another note somewhere that says "top of steel elevation is XXX unless otherwise noted." That beam is set 2.5" higher than the others. I'm going to guess and say that it runs parallel to a bunch of open web steel joists (probably with a K designation).
Probably means, as phamENG said, it's 2 1/2" deeper than the other beams, or deeper than some other option at that location.
Note: You may want to edit the text of your post to match the graphic, celeron8. (2 1/2" instead of 2 1/5") Before I looked at the graphic, I was about to move on, having no idea what +2 1/5" would be.
Rod Smith, P.E., The artist formerly known as HotRod10
This notation is used in erection drawings. In general the elevation of TOS (top of steel ) is given for each floor with general notes. The exceptions for each member is indicated by giving TOS distance (+) above or (-) below.
In this case , W 21X55 (+2 1/2in ) means, the TOS level for this member +2.5 inch higher than the general floor TOS elevation. For instance, if Elevation TOS =60'-2.5", the TOS elevation of W 21X55 (+2 1/2in ) member will be =60'-5".
