In the ole day, there were 4 levels of maintenance:
1. Operator (done by the crew)
2. Organizational (done by the unit mechanics)
3. DS/GS (direct support/general support)
4. Depot level (done in the US depots or by manufacturer for the most part although there were exceptions)
Direct support means the maintenance unit is attached to your unit to provide support directly to you whether it you be a battalion, brigade, division. These units tend to be somewhat smaller and normally support at brigade level. 3rd Bde, 8th infantry division and the 202nd Forward Support Battalion were directly supported by the 51st maintenance company. If the crew couldn't repair the tank, the battalion mechanics tried, if they couldn't the vehicle was evacuated to a maintenance collection point where if the rest of the battalion mechanics worked. If they couldn't repair it, a maintenance team from the B company of the forward support battalion (FSB) tried at that location and if they couldn't repair it, it went to a central point in the brigade support area where the rest of the B company FSB mechanics worked. If they couldn't fix it, then it could be turned over to the direct support maintenance company to do the really big stuff and if they couldn't then back to the rear with it and order a new vehicle.
General support means you provide maintenance to...say a geographical area. X unit is responsible for all maintenance support within the state of Maryland or on foward operating base henry or on Aberdeen Proving Grounds. These units normally operated above division level. If a unit does not have the capability to repair a vehicle, the GS maintenance guys take over.
The Army now has 2 levels of maintenance:
1. Organizational (which includes crew level)
2. Non-organizational (send the vehicle up the chain til it gets fixed)
Understand there are a boat load of contractors that now go to war within units. Stryker units have a LOT of contractors doing the maintenance work on what use to be DS/GS level.
That's about it in a nutshell in the 5 minutes I had to type this.