Certain aircraft have developed for them a Master Minimum Equipment List, or MMEL. Many operators or individuals seek to obtain approval for a MEL based on items taken from the MMEL, and the FAA will approve each one on a case by case basis. MEL items do not have to come from the MMEL, either; they can come from any reasonable source as long as they can be shown to be relevant and gain approval.
The MEL constitutes a supplemental type certificate. When an item on an aircraft is inoperative, and the aircraft does not meet it's type certification (an inoperative nav light, for example), it must be altered in a manner acceptable to the administrator. The MEL serves as a preapproved STC, or supplemental type certificate, which grants temporary authorization to operate under certain conditions with inoperative equipment. It changes the type certificate by supplementing it, and is therefore directly applicable to the airworthiness of the aircraft.
Because this is the case, the MEL is part of the approved aircraft documents, and is aircraft-specific. A fleet may be put on the same MEL and named, but each aircraft on the MEL must be named, and the MEL document in each aircraft is aircraft-specific.
You cannot pick up the MEL for another aircraft and use it for your airplane, nor can you use the MMEL unless that particular copy has been specifically approved for your aircraft. This is the case regardless of the regulations under which you're operating, be it Part 91, 121, 125, or 135 for that matter. You still need the approval.