something, usually a document, that is attached to a larger work as a supplement; an attachment
an addition to a building
An annex1 is distinguished from an appendix by the fact that it is created independently of the larger document.
Jenkinson 1937, 7The distinction between what can and cannot be “annexed” to a document is like all fine distinctions, difficult. Its particular difficulty may perhaps be illustrated by a reductio ad absurdum. Supposing for example that a Viceroy sends home to the Secretary of State in England an elephant with a suitable covering-note or label; or supposing, to take a more actual example, that the Government of a Colony presents to the First Commissioner of Works in London a two-hundred foot spar of Douglas Pine: the question may be imagined to arise: Is the spar “annexed” to correspondence with the Government of British Columbia? Is the elephant attached to the label or the label to the elephant? ¶ The answer to those who would put this dilemma to us in the present connexion is that the Administration would be obliged in all such cases to solve the question of housing—to send the spar to Kew Gardens or the elephant to the Zoo—long before the label or letter comes into charge of the Archivist: the problem is an Administrative, not an Archive one.