For my RTS..
I am storing the units that my player-client selects on the client and adding which units are selected to the action encoding message to the server.
However, I can make it optimal, so that each time a unit is selected or deselected, the server is informed, so knows what is selected by each client, so ctrl-clicking groupings is simple and units associated with an activity is simple too if the server knows what units have been selected.
It is always either a grouping or a selection that is moved. If hundreds of units are selected, possible, the byte would be huge.
Is this what pro RTS's do?