Fhirbase reads input files sequentially, reading single resource at a time. And because of PostgreSQL traits it's important if Fhirbase gets a long enough series of resources of the same type from the provided input, or it gets resource of a different type on every next read. We will call those two types of inputs "grouped" and "non-grouped", respectively. Good example of grouped input is NDJSON files produced by Bulk Data API server. A set of JSON files from FHIR distribution's "examples" directory is an example of non-grouped input. Because Fhirbase reads resources one by one and do not load the whole file, it cannot know if you provided grouped or non-grouped input.