Tubing that is run inside the production casing and used to convey produced fluids from the hydrocarbon-bearing formation to the surface. Tubing may also be used for injection. In hybrid wells, for example, tubing is used as a conduit for gas for artificial lift below a mudline pack-off tubing hanger to isolate the gas lift pressure from the production riser.
Source: API RP 90, Annular Casing Pressure Management for Offshore Wells, Upstream Segment. First Edition, August 2006. Global Standards