- "description": "External References are used to link entities to equivalent entities. The External Reference has two fields:\n\n* **referenceId** - The external reference ID. Could be a simple string or a URI.\n* **referenceSource**\t- An identifier for the source system or database of this reference.\n\nThere are two main uses of External References.\n\n* An array of External References are provided in the result of many queries, which allows equivalent entities\n to be found in other systems using these identifiers.\n* External References can be provided as arguments in many queries, which allows querying by\n\nThe value of the **referenceSource** field provides the context of the **referenceId** field. \n\nThe table below shows the currently available `referenceSources` for different entities, a description of the `referenceId`, and an example:\n\n| Reference Source | Entities | Description | Example |\n|---------------------|---------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------|--------------------------------------|\n| CROP_2_LETTER_CODE | Crop | The 2 letter code for the crop defined in the Crop Ontology | WH |\n| CROP_3_LETTER_CODE | Crop | The 3 letter code for the crop defined in the Crop Ontology | TRI |\n| CROP_ONTOLOGY_ID | Crop | The ID of the crop in the Crop Ontology | a9d1511d-a50b-3838-a3fe-58219510e6f4 |\n| PRISM_CROP_CODE | Crop | The crop code for the crop defined in PRISM | WH |\n| PRISM_CROP_ID | Crop | The business code for a crop in PRISM. It is in the format ^PCRP(?=FCR VEG)[0-9]{4}[1-9]? | PCRPFCR1004 | \n| PRISM_MATERIAL_ID | Germplasm/Cultivar | The ID used for Material, which can be used to uniquely identify Cultivars or to identify Germplasm belong to a PRISM material. | PMWH20743430 |\n| PRISM_MATERIAL_NAME | Germplasm/Cultivar | The Name used for Material, which might be the Cultivar name or the name of PRISM material associated with a Germplasm | G16Q2x0749.WTHCMSt |\n| PRISM_NFB_ID | Study/Locale | The ID of the Nursery Field Book in PRISM. Provided when the Study or Locale originates from a PRISM Nursery Field Book | PDCEFCR565640 | \n| PRISM_NFB_NAME | Study/Locale | The name of the Nursery Field Book in PRISM. Provided when the Study or Locale originates from a PRISM Nursery Field Book | | \n| PRISM_YTFB_ID | Study/Locale | The ID of the Yield Trial Field Book in PRISM. Provided when the Study or Locale originates from a Yield Trial Field Book | PDCEFCR565569 |\n| PRISM_YTFB_NAME | Study/Locale | The name of the Yield Trial Field Book in PRISM. Provided when the Study or Locale originates from a Yield Trial Field Book | |\n| TRAM_CODE | Observation Variable | The code of the equivalent variable in TRAM | TTOC:0005045 | \n\n### Implementation\n\nExternal References are implemented consistently across data providers (PRISM and Phenome),\nbut a Data Provider may not provide all possible External References for a specific Entity."
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