All editions, impressions and issues of a work are described separately. For the definitions, the term 'edition' is used as a neutral term, covering every printed presentation form, regardless of its inception with regards to content or technique.
All editions (copies) that were printed at one time from the same or largely the same 'formes' or type.
Part of the edition that contains all copies that came from the press at one specific time. In the period before 1800, edition and impression are practically identical, since the set type was almost always destroyed after printing. The terms in this period can be used interchangeably. We always speak of editions.
Part of the edition that contains all copies that were presented (offered for sale) as a deliberate publication-unit, different from other issues. All issues of an edition therefore consist of (largely) the same sheets. A change on the title page to correct a mistake does not result in a new issue but in a different state. A deliberate change in contents does result in a different issue (reissue) and thus a new bibliographical unit, which is described separately.
Stategeneral note. When a different state results in another fingerprint, both fingerprints are added. Deviations from the collation formula are explained in a general note. Different states may have different groups of readers in mind by, for example, the addition of a poem, a different piece of text, a different dedication, etc. These differences in content may be indicated in the copy-specific information.
Differences in state may also appear on the title page. If that is the case, it is best to mention these differences in a general note, to avoid confusion for the user.