I purposely kept this simple… No mention of GA vs Commercial, and no altitudes. I just wanted to outline my understanding of the various contact options.
Used when you want to fly your flight plan. This lets the controller know you will be following the path defined in the flight plan. In addition, you can fly at whatever altitude you feel appropriate. It is not necessary to ask permission to change altitudes!
Be aware that the controller may give you heading and/or altitude adjustments as he sees fit to keep you clear of other aircraft under his control. Once clear of any other aircraft, the controller will usually tell you to resume own navigation which is when you may resume flying your flight plan.
Check In (With You from the ATC standpoint):
Used when you will be following your flight plan, but this service does require that you clear any and all altitude changes with the controller. Just like Flight Following, the controller may deviate you from your heading and/or altitude to clear other aircraft.
Check in is also used when you’ve been transferred from departure to center, or center to approach. The service you requested remains with you, so there’s no need to ask again after switching to the new frequency.
Used when you want heading and altitude vectors to get you in the proximity of your destination airport. The controller will get you within range of the your destination airport, give you a final vector that will get you to pattern altitude, and will hand you off to tower. Tower will then give you pattern entry specifications, sequencing if necessary and clear you to land.
Used when you want the controller to provide heading and altitude vectors to your destination airport including final vectors that will set you up for an ILS approach to the runway specified. Once it’s clear you are going to intercept the localizer, the radar controller will switch you to tower. Once on tower frequency, you should call in ON THE ILS for the runway the radar controller vectored you for.
Here’s where I get in trouble… I believe this makes sense when you want to fly “around” say in the mountains, without a FP, but IRL who would fly without a FP unless doing patterns at your local airport. It’s not a way around ATC vectoring people into a busy airport. Like the other services, you still may be moved if you are interfering with approach or departure at a controlled airport.
Be aware that based on traffic density the controller may switch your requested approach service and/or requested runway if ILS.
If you’ve been granted “Check In” or FF or even radar vectors you may be switched to ILS if there is a stream of aircraft inbound for the destination airport.