The Alternate Air Source “Octopus reg” is an extra second stage connected to the first stage of a regulator comprising a major component of the Air delivery system. This “spare” regulator works exactly the same as your primary regulator second stage. Should divers run very low or even out of air they needto locate the Alternate Air Source from their dive buddy, quickly!
For this reason, the Octopus regulator is usually marked with bright colours, like yellow or another bright colour.
Generally, it is best to use an alternate air source and primary regulator from the same manufacturer. This is important because the primary first and second stage's intermediate-pressure and low-pressure port sizes, position and intended use vary.
It is even more important if you select an alternate inflation regulator because it must match your BC's low-pressure hose and ambient corrugated hose, while still allowing the remote exhaust to work.