If you are doing a normal Category I ILS approach, flown to a DA (decision altitude) of 200 feet above threshold elevation with QNH set (for example, threshold elevation 190 feet AMSL [Above Mean Sea Level], DA 390 feet barometric altitude for a DH [decision height] of 200 feet), then you may well set the "bug" on the radar altimeter to "200" for an additional alert to reaching your DH. That radar altitude of 200 feet is not actually part of the Cat I ILS but just an additional thing you may wish to use, if it is available.
Approaching over uneven terrain the RA will, of course, be fluctuating and so not be of much practical use.