What does the F stand for in dirt bikes?

In the realm of dirt bikes, terminology often carries a specific significance, and the use of abbreviations and acronyms is quite common. As a motorsports enthusiast, I'm curious to know: What does the 'F' typically stand for in the context of dirt bikes? Is it referring to a specific frame design, a certain feature, or perhaps a brand name? Clarifying this abbreviation could help us better understand the intricacies of dirt bike specifications and terminology.