Empty: if you want to build entirely your own system
Units & FP: when you want to use different elementary flows but can work with the units and flow properties at least; better when importing anything because otherwise even the units need to be mapped.
Both are less common used I think than creating the database with reference data, but we wanted to provide a way to work also independent from our system, and not use "hardwired" categories mandatorily, as some other LCA systems.
