Exactly as it sounds: a piece of a specification which, for whatever reason, has not been published as a physical part of the main document body. An addendum does not stand on its own, but is part of the specification, and its provisions carry the same force. Examples of addenda include classified material (which can be either entire requirements or just the numerical portions), or requirements for a specific distributed subsystem, or requirements being allocated to some individual class of CI.
An addendum is always called from the main specification, even if the call is from the Classification section (see MIL-STD-490, “GENERAL REQUIREMENTS FOR SECTIONS OF SPECIFICATIONS” or MIL-STD-961 “Classification and Part or Identifying Number (PIN)”). No matter what the rationale for an addendum, it never replicates the allocated contents of the main specification, which would entail the possibility for internal contradiction. In this context, “allocated contents” could mean that the text of individual requirements is repeated, but the allocation is to some other class of CI. For example, if the computational/communication network for a system was sufficiently complex and extensive, its requirements might be published in an addendum, as called from the Performance Requirements section of the main body.
Also note that an addendum is not another name for “appendix”, which is published as part of a document. Like the main body of the specification, an addendum can have appendices.