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.[ You can opt out of Google Ads if you are a registered user at /ad-options ]
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