The coordinate system that should be used throughout GA4GH standards is 0-based half open.
So in a sequence of
… the exact match for GTC would be
start: 3, end: 6
… due to interbase coordinates:
A C C G T C G A | | | | | | | | | 0 1 2 3 4 5 6 7 8
A detailed documentation of the standard can be found on the SchemaBlocks site.