Ok, so it's swapping between the shown units.
http://imgur.com/vKd3yBNIn this screenshot, I can swap the selected units between the 2 units in reserve with the blue arrow. When I right click on the grey flag (to the left), it's the same results (with automatic choice). When I left click on the grey flag, the flag becomes blue, nothing happens. I don't know what is the purpose of left click on it.
Also, I can set a reinforcment to all units with some little things like artillery spotter, MG or AT gun. When I click on the button to the right of "306 ID" green line or the "1 infantery battalion" blue line, it's the same results (with automatic choice).
When a red unit can be sent to the reserve, a second button appears to the right. It send the red units to the reserve automatically (if I understood correctly, it's the same results when we make that manually).
Some questions:
What is the "reserve" list to the right of the "306 ID" green line? (reinforcment 2, heavy weapon 12, spotters 3, guns 4, transport 2)
Is it the list of the shown units, or the list of reinforcment, or a list of future and hidden units?
The red units sent to the reserve will be treated and fixed in background or they will remain as is?