Hi

Do not understand your question. ATX is the form factor (dimension of mobo, postiion of mounting holes and type of powersupply) ATX is the most common form. There a number of different form factors out there, most commonly being eATX (extended ATX, a bigger sized mobo) and mATX (micro ATX, a smaller mobo) but they can go down to something like picoITX a very small motherboard.

Size and form factor. That's what it implies.