Generally speaking, that denotes the engine size for that aircraft.  Not withstanding, though, that is usually in the order of a recommendation for general flight.

If you take a 40 size plane, they will often recommend a range, say .40 to .55 two stroke, or .50 to .65 four stroke (not that such beasties exist, but it's an example).

That usually goes with the type of plane, the airfoil arrangement, wing span weight, and so on.  A good example of this is the Sig Kadet, a trainer with a 70 inch span takes a .30 to.40 engine where their Four Star 60, a low wing itermediate level aircraft, has a 71 inch span but takes a .60 to .75 size engine.

Hope this helps.

CGr.