What's the action to make a circle below a unit? I didn't know what to search since I don't know what's its called..

Transmission does it.

You can have 3 different Circles. Red, Yellow, and Green.
Green is for your units. Yellow is for either Neutral players or Allies of the person receiving the transmission, and Red is enemies.
You CANNOT Make a green circle to a unit of your own and have yourself not see that unit via Losing vision to yourself. I learned that the hard way. There is one exception, and that is with a cloaked battlecruiser.
You CAN have circles that are Red or Yellow that are of cloaked (non-visible) units.

So you can't have the circle appear a visable unit?

You CAN have circles that appear on visible units.
I'm just adding the fact that you cannot have circles on non-visible units that are owned by the player receiving the transmission WITHOUT taking off the cloak.

It works, thanks.