USG Handbook.
Level 0 Used in temporary construction or wherever the final decoration
has not been determined. Unfinished. No taping, finishing or
corner beads are required. Also could be used where non-predecorated
panels will be used in demountable-type partitions that are to be
painted as a final finish.
Level 1 Frequently used in plenum areas above ceilings, in attics, in
areas where the assembly would generally be concealed or in building
service corridors and other areas not normally open to public view.
Some degree of sound and smoke control is provided; in some geographic
areas, this level is referred to as “fire-taping,” although this
level of finish does not typically meet fire-resistant assembly requirements.
Where a fire resistance rating is required for the gypsum board
assembly, details of construction should be in accordance with reports
of fire tests of assemblies that have met the requirements of the fire
rating imposed.
All joints and interior angles shall have tape embedded in joint compound.
Accessories are optional at specifier discretion in corridors and
other areas with pedestrian traffic. Tape and fastener heads need not
be covered with joint compound. Surface shall be free of excess joint
compound. Tool marks and ridges are acceptable.
Level 2 It may be specified for standard gypsum board surfaces in
garages, warehouse storage or other similar areas where surface appearance
is not of primary importance.
All joints and interior angles shall have tape embedded in joint compound
and shall be immediately wiped with a joint knife or trowel,
leaving a thin coating of joint compound over all joints and interior
angles. Fastener heads and accessories shall be covered with a coat
of joint compound. Surface shall be free of excess joint compound.
Tool marks and ridges are acceptable.
These definitions are similar to, but not identical to, ASTM C840 (not referenced in IRC).