it's fully gui (only the point/group or other leaking variable removings, which use custom script).
well, first you have to make a model (a small square)
then you to make an icon, you have to create a new destructable (best option is a pathing blocker).
then you change the replaceable texture id of it, to your icon path.
then place it and you have your icon. Now you cannot select this destructable (not with triggers) so you have to create a new unit. I use the same model for the unit as for the destructable. now you place the unit below the destructable so that you can't see it. Then you can trigger it. Just use the event: A player selects a unit, if it's the unit under the destructable, then do your triggers.
It's hard to explain.
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