To retrieve information about specimens from a database, every specimen must be uniquely identifiable. Collectors' names and field numbers are not always reliable for this because some collectors have the same names and some collect with duplicated field numbers.
For the database to work properly, each specimen is asssigned a unique accession number which distinguishes it from all other specimens in the collection. This unique number mabe found on the herbarium specimen sheet or may be on the specimen label.
The combination of the accession number and the unique institution code from Index Herbariorum uniquely identifies a specimen from any other anywhere in the world.