Another example is placing a cheap unit or a submarine in the water on Infinity Isle where potential harassment-moves such as Cryos or TwinFrogs are coming in from. That way you will know about it when it's coming and buy time to prepare.
To formulate it more clearly:
Through aim-oriented scouting you check if something already happened such as a tech-up, an expansion, production of a type of units.
Through monitoring scouting you supervise when something you might have predicted eventually happens such as harassment coming in, or a Refinery goes up which you plan to harass instantly.
Logically, you can use an originally aim-oriented scout and put it in a location it monitors the area, given that the unit didn't die in the process of the first scouting task. On the other hand you can use a monitoring scout to check something. The two forms of scouting are transformable.
In my eyes players underrate the value and benefit that arise from intelligence. It's hard to come out ahead from a fight through micro. Through scouting you can compose your army better and with that make your army stronger while not bigger resulting in you having advantage before a fight even starts.
Observation outposts are also highly underrated!
As scouting is so important and information is so valuable, the mechanics of anti-scouting are a separate story and a layer of skill in RA3.
Map-control means controlling certain areas of the map in a way you make it harder for your opponent to pass that area. Consequently, wherever units or turrets of yours are the area is under your control - control meaning that your opponent will have to invest both units and time to pass through the area.
In the very beginning of a game there will usually be a run for the garrisons and the Oil Derricks. Whoever gets there first wins that race in most of the cases. The winner of that race is depending on who
timed build and unit order better as well as who supported the garrison occupants - respectively the Engineer - better.