Execution connections are connections to windows servers that exMon is installed on. By default you can only have one execution connection per windows server.
For standard Execution Connections you install exMon on the Windows Server and an Execution Connection is created for the server.
You can have multiple UATs for an Execution Connection, it requires installing exMon to two Windows Servers, opening exMon Administrator and merging the details of both Execution Connections to the same Execution Connection under different UATs.
This can be helpful if you have TimeXtender or other command line packages running DEV and PROD on different machines.
Read about Installing Execution Connection