I think this particular one (ROA) is an octave coupler on the recit division. There will also be a unison coupler, and at least one of them needs to be engaged in order for the recit to sound. Depending on which instrument it is, there may well also be a unison coupler on the Grande Orgue, and quite possibly a GO sub-octave coupler labelled something like "GO Octaves Grandes". Again, one of the GO couplers will be necessary for the division to sound.
There are also usually couplers for Recit to Positif, Recit to Grande and Positif to Grande controlled by pedals. The couplers onto the pedals (R-Ped, Pos-Ped and GO-Ped) are called Tirasses.
Finally there are the Pedals du Combinaisons. These control ventils which shut off the wind supply to some of the wind chests. Each division is on a divided wind chest, with one section, referred to as Fonds, that is always on, and another section, referred to as Anches, which is only on when the relevant combination pedal (or in the case of the Positif division of Metz and Caen, combination button) is activated. The Fonds are the flues at 16', 8' and 4' pitch (and in the case of the Recit, the 8' Basson-Hautboy), and the Anches are the upperwork and reeds.
To get a Cavaille-Coll type crescendo, couple all the manuals together, start playing on the Recit, add the anches, then go down to the Positif, then the GO, then add the Positif Anches, then the GO Anches, and finally the Pedale Anches, which are very loud. You would probably start with the Recit Tirasse engaged, and add the other Tirasses when you add the GO Anches. French music written for CC organs usually has pauses, or at least sections where the relevant foot is free, so that you can do this seamlessly. It is rather different from a crescendo on a UK organ where you are working through the divisional pistons.