I can't seem to find reference to this in the 90.1 User Manual.

In Appendix G, Table G3.1.1A, there is a reference to a Fossil/Electric
hybrid for determining Baseline system type. Is this a system that, for
example, might have a gas-fired RTU with electric VAV boxes?

In other words, if there is ANY gas heat in your Proposed building, your
Baseline building would have to automatically default to a fuel furnace
or gas-fired hot water heating system, correct?

Thanks,

James Hansen, PE, LEED AP