Per Wikipedia: Small form factor (abbreviated: SFF) is a term used for desktop computers and for some of its components, chassis and motherboard, to indicate that they are designed in accordance with one of several standardized computer form factors intended to minimize the volume and footprint of a desktop computer compared to the standard ATX form factor.
SFF PCs are highly desirable in the fighting game space as they often take up far less space than game consoles while still being able to provide acceptable gaming experience for emulators and less demanding games. This page will list some models known to be in use by organizers.