Of Occurrences, Accessions, Catalogue Numbers, and Collection Objects

Terminology in natural history collections can be surprisingly varied. At its core, Collection Objects are simply the physical items you curate - vouchers in a Herbarium, seed packets in a Seed Bank, DNA aliquots in a DNA Bank, and the living plants themselves in a Living Collection.

Beyond these “Objects”, you’ll also encounter terms like Occurrences, Accessions and Catalogue Numbers. In the sections that follow, we’ll unpack each of these and see how they fit into the wider world of collection management.

Occurrence, Accession Number and Catalogue Number

Occurrence

Natural history collections share one more thing: they generally all work with the concept of Occurrence.

An Occurrence, according to GBIF, is:

"a record of a taxon observed at a specific location and time"

In other words, an Occurrence is the evidence of an organism (WHAT) in a specific place (WHERE) at a specific point in time (WHEN).

In Living Collections, a related term is wild provenance, since the occurrence links to where a collection object comes from. Plants derived from nurseries or other cultivated origins might lack wild provenance and therefore have no Occurrence, or an Occurrence with no wild location data and a cultivated flag.

Accession Number

The Accession number is a more flexible concept. According to BGCI, an Accession is:

“plant material (individual or group) of a single taxon
and propagule type with identical or closely similar parentage
acquired from one source at the same time.”

This definition means an accession can encompass:

  • Clonal material, where every propagule shares the same genotype.
  • Sibling material, such as seeds harvested from a single parent.
  • Population samples, for example seeds or cuttings collected from multiple individuals in the wild.

That breadth is both an advantage and a challenge. On the plus side, it lets institutions group related material by provenance, genotype or collection event under a single accession. Conversely, without consistent guidelines, one garden’s “accession” may differ in scope from another’s, making data exchange and interpretation tricky.

Sometimes the lines between Occurrence and Accession can blur. Some institutions might decide to unify both concepts in one, like in Kew, or to keep both separate. In general terms, we could say:

  • An Occurrence record captures wild provenance, wild collection details and taxonomic data.
  • An Accession in a Living Collection groups related plants according to shared provenance, parentage or wild collection event.

The Accession Number is typically composed of the consignment year - the year the material entered the collection - followed by a sequential identifier. For example, accession 2025-105 denotes the 105th accession created in 2025.

Catalogue Number

Because multiple Objects can share the same Accession Number, each individual Object also receives its own Catalogue Number. A Catalogue Number is usually formed by appending a unique suffix to the Accession Number.

In a Living Collection, an Object record uses the Catalogue Number as the identifier and can store data such as a plant’s location within the garden, its current status - dead or alive, or phenology details, to distinguish one specimen from another.