In Starcraft, each unit is assigned to a particular player by color. For example, all units with blue team colors belong to the blue player.
When placing units in the Campaign Editor, you can use either the Tool Tree or the Brush Palette:
For more information on placing units, see Placing Units.
To change a unit’s owner, select the unit or units you want to reassign and press a number key corresponding to a player. The units will change ownership to that player. Alternatively, you can use the Player menu instead of a number key. You can also change unit ownership in the Unit Properties dialog.
Start Locations cannot change ownership.
You cannot assign a unit of one race to a player of a different race.
You cannot assign a unit to a player if it would cause that player to use more than 200 Unit Slots.