What is the difference between FLAT FHIR and FHIR Bulk Data?

FLAT FHIR, FHIR Bulk Data, and Bulk FHIR are all one and the same. The official HL7 standard (that we led balloting for) is called FHIR Bulk Data. But it's synonymous with FLAT FHIR (coined by Boston Children's Hospital SMART team), and Bulk FHIR.

Here is more information on FHIR Bulk Data / FLAT FHIR / Bulk FHIR​