Over the course of his long career, Vagn Holmboe wrote close to 400 separate works. He appears to have been fairly conscientious about cataloguing them himself, which is undoubtedly unhelpful.
During his maturity, Holmboe used the traditional opus numbers to list his works. There are 197 opuses in total, the last being the string quartet that Holmboe was working on at his death which was completed by a distinguished pupil. In contrast to some other composers, the opus number is generally a very good guide to when Holmboe composed a piece. It is also the number that is most commonly used on recordings, and so it is the primary method used in this discography along with a work's title.
There are, however, a few things to take note of. When an opus consisted of more than one work, Holmboe would use letters after the opus number to distinguish them. So, for example, the Two Border Ballads op.110 consist of two discrete works for choir, A Lyke-Wake Dirge (op.110a) and The Wee Wee Man (op.110b).
The relationship between works within an opus varies. Holmboe would sometimes reuse the same opus number when he undertook a major revision of a work. For example, the originally published version of the piano piece I Venti is now labelled as op.99a, with the revised version from a few years later known as op.99b. Holmboe also used the same opus number when arranging a work for different forces - op.103b is a version for choir of the same music as op.103a for chamber orchestra.
On some occasions Holmboe labelled a work with 'a' because he intended to add a 'b', but the second work did not eventuate. So for example Beatus vir is op.96a but there is no op.96b.
Not all of Holmboe's works have an opus number. There are works from his youth, which predate his decision to declare his career as a professional composer fully established with his opus 1. There are also pieces that he did not assign an opus number to, such as works written for a specific occasion, for children or for particular amateur groups.
To deal with this, Canadian musicologist Paul Rapoport developed a system of Meta numbers which comprehensively catalogues all of Holmboe's works, as far as possible in chronological order of initial composition. Occasionally this means the sequence is not the same as for opus numbers. The 1996 version of Rapoport's catalogue, The Compositions of Vagn Holmboe (published by Wilhelm Hansen), can be regarded as definitive for all but a few compositions and is a primary source relied on here.
Meta numbers are not used as often on recordings. The BIS record label, though, has made use of them alongside opus numbers, and they are provided in the discography as an aid to identifying a piece.