Conduit fill is the percentage of the area inside the conduit taken up by the cables. Consult applicable codes, industry standards, and manufacturers' data for further information on conduit fill.
Dimensions and percentage areas of conduit and tubing are typically provided in tables. Dimensions for various types of conduits can be found e.g. in Chapter 9 of the 2005 National Electrical Code. Cableizer indicates a warning if the maximum fill percentage is above the NEC limitations of 53% for one cable, 31% for two cables, and 40% for three or more cables. Cableizer cannot be held responsible whether or not your arrangement is within regulations.