GCC6809 supports several different calling conventions: ways of using registers to make function calls. You can explicit select one via the -mabi_version option, or you can take the default, which will generally be the most efficient method implemented to date.
If developing a standalone program that doesn't interact with any libraries or precompiled sources, the default is probably the best option. However, when integrating with existing code, you probably need to use the conventions already in place.