 It uses a subset of the motherboard mounting holes required for microATX and the same I/O plate system as ATX and microATX.FlexATX specifies that a motherboard be no larger than , and can have no more than three expansion slots.The term is used also for the form factor of a PSU that is smaller than a standard ATX PSU and is used in small cases that host a FlexATX or Mini-ITX motherboard or in thin rackmount servers such as 1U racks.