This should be used to store a unique identifier for the taxon from an external database. For example, in the UK we have a national species list called UKSI that is managed by our Natural History Museum. This has 16 character unique identifiers for all species names so we use that - meaning it is easy to cross reference species lists in Indicia against UKSI and it is also easy to cross reference between different species lists within Indicia. We normally store the identifier for the currently preferred name for that species in this field so that all synonyms and common names have the same external key.

So - if you are getting your species names from some other database (Species 2000 etc), then you might like to use the unique IDs from that database for easier cross referencing. If you are authoring your own lists straight into Indicia then you might not need to fill this field in at all.