The Lowest Replaceable Unit (LRU) is defined dependent on the operating situation of the design.  It represents the lowest ambiguity group which the diagnostics will isolate to for the situation that the design is in.  It might be refered to as a Factory Replaceable Unit (FRU), a Mission Replaceable Unit (MRU). or many other Industry used teminologies for an item or group of replaceable items. For example a set of defined Factory Replaceble Units (FRU's), [not dependent on down time] may be totally different than a set of defined Mission Replaceable Units (MRU's), [totally dependent on down time] for the same design. In eXpress the LRU is normally defined utilizing the the Operating Modes and/or attribute checklists.