Elem is a filenaming nickname for the stream load element of interest - "bicarb" for "bicarbonates", for instance.
ElemNum is a list of USGS constituent code(s) for that element, to extract from the quality data file. This is a simple comma-separated list of 5-digit code numbers, such as:
99405,99113,00945for a possible query on filtered sulfates.
By putting codes into the same list, you're in effect saying the quantities are "the same thing", to the extent that 10 units of one, are equal to 10 units of the others. It's up to you to determine whether that is valid for your context.
Note that the stream flow values used in the calibration phase are 00060 and 00061 by preference (but if they aren't provided in the calibration data, LoadRunner uses values from the flow file, or interpolates them.) This behavior isn't changeable from the user interface.
Unit Conversions : It is also possible to specify a unit conversion on some of the USGS codes in your ElemNum list. This implies that code XXXXX measurements are equivalent to YYYYY x conversion factor. For example: USGS 00915 is Ca, in mg/liter. USGS 00910 is CaCO3, in mg/liter. But CaCO3 is 40% Ca by molecular weight. So you could have a LoadRunner run specifying:
This says to take every measurement of 00910 and multiply it by 0.40.
This could also be used to combine measurements in milligrams/liter with those in micrograms/liter. Be careful when changing orders of magnitude, though - the calibration file uses only 3 decimal places. So if AAAAA is in milligrams, and BBBBB is in micrograms, you'd get more accurate results using a list of "AAAAA,BBBBBx1000.0" (as opposed to "AAAAAx0.001,BBBBB", which could lose three significant digits).
Please note that LoadRunner knows nothing about your units. The comments on the output files suggesting "kg/day", etc. all assume your input concentrations were in mg/liter. If they were actually in micrograms or other, please translate accordingly.