Each IOCtl function has a category and a function code. The category defines the type of device to be accessed. OS/2 has several predefined categories. In general, all codes in the range 0x0000 through 0x007F are reserved for predefined categories. A device driver can use additional categories, but they must be explicitly defined by the device and be in the range 0x0080 through 0x00FF.

In each category, a function code defines the action to carry out, such as reading from or writing to the device and retrieving or setting the device modes. The number and meaning of each function code depend on the device driver and the specified category.