It's connected to full face mask to on-demand 100% O2 outlet. I guess it can be used as reserve for normal crew O2 masks at any level for period specified in FCOM (depending on consuption depending on activity level from 7 to 71 min). I dont know what else could it be used in this configuration.
Also I was told (not confirmed) that cabin portable O2 masks can be connected to other (constant 3 litre flow) outlet. I would like to know what would be situation to use bottle in this config. I'm worried that O2 partial press would not be enough for all levels decompressions.
Finaly what would be procedure if for some reason crew O2 press. is depleted during the flight.