RAM Concept allows you to work with three unit systems: US, SI and MKS.

Some designers refer to the US units system as "US customary units", and others call it "Imperial". SI and MKS are metric unit systems, with MKS using mass rather than weight.

It is up to you which system you use but local practice should dictate your choice.

The choice of actual units is more subjective. For example, after choosing the US system, one designer might use the default area load units of pounds per square feet, and another might change the selection to kips per square feet.