ITX is a case form factor ie the standoffs for mounting a motherboard for each form factor of the motherboard are in a specified location and distance on the motherboard tray or bracket.
The smallest form-factor being, Thin ITX, then ITX, DTX, then MATX, then ATX, then extended ATX the largest. ITX stands for Infomation Technology Extended.
For the most part, this site will cater to Thin ITX, ITX, and DTX as most builds are about reducing case size and total volume. However, there are case designers such as
such as SFFTime and Custommod who have case designs supporting very small volumes and ATX board support that showcase their work on this site.