It is not derived from something, it is specified by you in the Quatity field in the Dialog.
If I remember correctly, you specify how much or what quantity of that component is per reference quantity. The reference quantity can be some length or surface or volume. So if you specify "Surface A" as reference quantity, then you need to provide in the Quantity fields how much of that component is in 1 sqm of surface. For example, how many bricks there are per square meter of Brick Wall built. (But that quantity could also be based on the volume of the brick wall, not the surface of its left side or right side).
ArchiCAD will then during listing calcuated the surface of each Wall then multiple the quantity you specified with the wall surface and thus obtain the number of bricks in that Wall.
But I need to tell you that this is kind of a legacy feature in ArchiCAD, it has not been developed in the last 6-8 years at least, development focus was shifted to the Interactive Schedule functionality.
However, there are still things you can do in this "old" listing of ArchiCAD you still cannot do in Interactive Schedules, but Interactive Schedule is much easier to use.
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