If you record a macro of you selecting those columns you get:
Range("A:C,E:F,L:L").Select
from which you can see where the commas go. Duplicate that.
Add a new sheet, put something in C10 and something in F3, then in the Immediate pane (Ctrl+G if you can't see it in the VBE) type:
Activesheet.usedrange.select
then look at the sheet. This should give you a good idea of what it is.
Excel's own help says: "Returns a Range object that represents the used range on the specified worksheet."