I'm not sure, but I think you might be asking how to desribe what gear you're in either to a riding buddy or to a mechanic when diagnosing a problem. To be honest, I never spoke in terms of first gear, second gear, etc.
The biggest differentiation I use when riding with my wife or with someone who is new to having so many gear choices is which chainring they should be in. I'll say, " Keep it in your middle chainring and pick a lower gear for this upcoming hill..." or, " You'll need to drop down into granny gear for this one..." or, " Go ahead and use your big chainring for this stretch of road...". Since the selection of rear gears is so individual, I always let the rider sort out what gives them the most comfortable cadence. If I try and say, "Put it in third gear..." or something comparable it always ends up confusing or frustrating the other person. As for myself, I use the orange indicater in the shifter window to gauge where I am. I don't correlate it to any specific gear number, I just use it as a reference to where I am on the rear cog and front ring. By now, I know what chainring/cog selections make me the most efficient in given situations so I just glance down and eyeball things in the shifter windows and go from there.