The letters RM or NM on a champagne label

The registration number is preceded by two letters specifying the professional category of the wine producer and marketer.

Current : 

RM: récoltant manipulant, who, using grapes from his own estate, produces and sells champagne under his own brand name.

RC: cooperative harvester, who works his vines and harvests his grapes, but does not make his own wine. He entrusts the grapes to a handling cooperative, which in turn sells him the wine to which he affixes his name.

NM: négociant manipulant, who buys the grapes to complete his harvest, makes the champagne on his own premises and markets it. Almost all the leading champagne brands have NM status.

Less common :

MA: buyer's brand or auxiliary brand, when the champagne is produced by a négociant who sells it to a distributor (supermarket, luxury store, restaurateur, etc.), who resells it under his own brand.

CM: handling cooperative, which produces and markets wines made from its members' grapes under its own brand name.

SR: société de récoltants, a group of winegrowers (often from the same family) who combine their efforts to vinify and market their production.

ND: négociant distributeur, wine merchant, who buys bottles on which he affixes his own brand.

Another concept:

In the French wine world, there's another important concept to consider: the winemaker's membership of the Fédération des Vignerons Indépendants. This membership guarantees that the wine is produced entirely by the winemaker, from vine to bottle, and can be easily identified by the corresponding logo.