This short discription can be found at here https://bcl-lab.github.io/FHIR_CG_web/sequence.html#resource The plain text should be: When saving the variant information, the nucleic acid will be numbered with order. Some files are using 0-based coordiantes (e.g. BCD file format) while some files are using 1-based coordinates (e.g. VCF file format) . The element coordinateSystem in Sequence Resource contains this information. Sequence.coordinateSystem constraints within two possible values: 0 for 0-based system, which will mark the sequence from number 0, while 1 for 1-based system, which will begin marking the first position with number 1. The significant difference between two system is the end position. In 0-based system, the end position is exclusive , which means the last position will not be contained in the sequence window while in 1-based system, the end position is inclusive , which means the last position is included in the sequence window. Note both systems has an inclusive start position. For example, ACGTGCAT will be numbered from 1 to 8 in 1-based system and will be numbered from 0 to 8 in 0-based system to mark flanks (i.e. place between two Nucleotide). So the interval [3,5] in 1-based system is GTG while interval [2,5) in 0-based system is same segment GTG. Besides, there is one slide (see attachment) that can be further implemented into Implementation Guide if people need better , clear exapainlation of coordinateSystem. Also, we provide two simple example (ideal case) to use both coordinateSystem. Check preview of two examples at: https://bcl-lab.github.io/FHIR_CG_web/coord-1base-example.html https://bcl-lab.github.io/FHIR_CG_web/coord-0base-example.html