> ## Documentation Index > Fetch the complete documentation index at: https://docs.isometric.com/llms.txt > Use this file to discover all available pages before exploring further. # Supplier Reference ID > Externally defined IDs supplied by clients that allow suppliers to query data using IDs from their own systems Most resources in Isometric's system require an externally defined ID supplied by the client, for instance an ID from the supplier's database. This allows suppliers to query data by an ID stored in their system instead of having to store Isometric's internal ID. This parameter is called the `supplier_reference_id` and should be specified in POST queries. The `supplier_reference_id` can also be used in queries to GET endpoints that return multiple objects. We do not currently support getting single objects by supplier reference ID. For a given supplier and resource type, the supplier reference ID must be unique and the POST query will return an error if it is not.