Definition of "Item Levels"The Joint Electronics Type Designations System (JETDS) and the Aeronautical and Support Equipment Type Designation System (ASETDS) both class the items covered by the respective system in categories like parts, units, groups, sets, systems, etc. These so called Item Levels are defined in MIL-HDBK-505 (formerly MIL-STD-280). Because different designation schemes are used by JETDS and ASETDS to designate items of different levels, the following rough explanation of the most important terms might be helpful.
Part: One piece of equipment, which has no function of its own. An assembly of several
pieces, which cannot be disassembled without destroying it, is also considered a "part".
(Sub-)Assembly: A combination of parts and/or other (sub-)assemblies, in which the components can be replaced/removed separately. The distinction between assemblies and subassemblies is essentially arbitrary. A (sub-)assembly is not used independently from other equipment.
Unit: A combination of parts and (sub-)assamblies, which is capable of independent operation on its own or can be used as a component of groups, sets or systems.
Group: A collection of (sub-)assemblies and units, which performs no independent function, but which can be part of a sets or systems.
Set: A combination of one or more units, together with associated groups, (sub-)assemblies and parts, which performs an operational function.
Subsystem: A combination of sets, groups, units, etc., which forms a part of a system.
System: Combination of several subsystems, sets, etc. which work together to perform one or more operational functions. The components of a system may be physically separated.
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