I am aware that we have 5 License Units on Community Edition. But I have issues figuring out how it's working.

I have Community Edition

USER>write $system.License.KeyCustomerName()
InterSystems IRIS Community

Freshly started system, only terminal session open, so, only one license units used, and 4 left. As expected

USER>write $system.License.LUConsumed()
1
USER>write $system.License.LUAvailable()
4

Quote from documentation - $SYSTEM.License.MaxConnections() returns the maximum number of connections a user can make while consuming one license unit. 

USER>write $system.License.MaxConnections()
25

2 12
1 200