Hi, for your own model, you do not need to model in USD but you can also use mass or another physical unit (assuming you link from one of your processes to the next; if you link to PSILCA background processes, it is good to use price). Of course USD works as well, so if you want to reflect that you have a mix of 55%/35%/10%, then 5.5, 3.5, 1 USD is ok, you should make sure that the output of the process fits to the scale of the inputs (so, if the input sum is 10 USD, the output should not be 1 USD probably if you do not have terrible losses in your processes).
Hope this helps! Best wishes, Andreas