From DOC 8168:
5.4.3 Outer marker/DME fix
5.4.3.1 The final approach area contains a fix or facility that permits verification of the glide path/MLS elevation angle/altimeter relationship. The outer marker or equivalent DME fix is normally used for this purpose. Prior to crossing the fix, descent may be made on the glide path/MLS elevation angle to the altitude/height of the published fix crossing.
5.4.3.2 Descent below the fix crossing altitude/height should not be made prior to crossing the fix.