Straight fueling is using Hydrogen solely as a fuel.
Dual fueling is using Hydrogen in any device that can also use another fuel solely - but neither is used at the same time. An example might be a small engine, or a natural gas or propane fueled home appliance like a furnace, hot water heater or clothes dryer
Bi-Fueling is where two fuels are burned at the same time and usually this is done in a larger engine with a gaseous mixer as a "vapor" device often called "fumigating" like in a Diesel or under a heavier load to consume less of the hydrocarbon fuel which is usually the "purchased" fuel using a twin fuel injection system setup.
There are laws which affect what you power with Hydrogen, but not how you make it so long as "greenhouse gases" are not created in the process
See also https://hpcman.com/content/which-design-hydrogen-generator-best-job