The Light Transmittance figures used in a Part L calculation come from two separate locations in Hevacomp. The best resource to distinguish the difference between these is the Data Reflection Report.
The first location Transmittance is entered is the Design Database Glazing menu - if this type of glazing is used in the building then this will be displayed in the list of Window/Rooflight constructions as below:
The Transmission factor can also be entered into Part L under the Miscellaneous > Additional Surface tab. Changing this will not affect the Window/Rooflight constructions above, but only the individual Window in that has been selected from the list of surfaces. This will then be displayed in the Zone (Room) properties, as below: