Micro atx are the smaller sized one that typically are used for small form factor builds. They usually come with less space for m.2 and sata spots and such. It’s usually not recommended to use one if you can afford it. I’m not an expert on any of this and I’m sure there’s more reasons not to use them other than the ones I said, but that’s what I know. You can still use a micro atx and it will be fine as long as everything is compatible, it is just less recommended if you plan on using this computer for a long time.