You need to choose the measurement system, e.g. points or picas, didots or millimetres, or whatever else is supported.

If you choose points, you need to select either "big points" or "standard points".

In United States, it is likely you may want to choose "big points" for new systems, and when upgrading from GN3, select whatever you've been using there.

In Europe, you you may want to choose "big points" as it simplifies sharing data with Adobe applications, but when upgrading from GN3, select whatever you've been using there.

See

Configuring Measurement System (chapter)