Refer to figure.
Learning Objective 062.02.05.04.01: Explain that ILS approaches are divided into facility performance categories defined in ICAO Annex 10.

ICAO annex 10 sets out facility performance categories in order to set out the requirements for CAT I, II and III approaches. There are therefore facility performance category I, II and III instrument landing systems.

The differences between the three facility performance categories are numerous, everything from the accuracy down to the monitoring is defined for all three facility performance categories, with many more variables in between. In the end, that means that they can provide increasingly good guidance to aircraft as the category goes up, and the associated minima get lower. As you can see in the annex above, taken directly from ICAO annex 10, facility performance category I can give guidance down to a height of 100 ft or less, category II down to 50 ft or less and category III down to the runway (with the aid of ancillary equipment).

It is important to note that the facility performance categories do not use the same altitudes at the ILS categories we use in normal operations. A facility performance category I ILS system is capable of providing information for a CAT I ILS as it gives useful information down to 100ft, which is lower than the lowest minimums of a CAT I ILS at 200ft. This gives some safety margin in the system, which is always useful at such extremely low heights.