Documentation: add description when it is DU, CU, gNB

doc/F1-design.md

@@ -116,6 +116,14 @@ sudo cmake_targets/ran_build/build/nr-softmodem --sa -O ci-scripts/conf_files/gn
 These files are tested in the CI, and are configured for use in docker,
 see [this `docker-compose` file](../ci-scripts/yaml_files/5g_f1_rfsimulator/docker-compose.yaml).
 
+The rules to decide if a config triggers a start of a DU, CU, or monolithic
+gNB, are, in order:
+1. If the `MACRLCs` section lists `f1` as northbound transport preference
+   (`tr_n_preference`), it is a DU.
+2. If the `gNBs` section lists `f1` as a southound transport preference
+   (`tr_s_preference`), it is a CU.
+3. It is a (monolithic) gNB.
+
 ## Configuration of F1 IP/port information
 
 For a local deployment, you should update the following fields.