Glashütte Rule is a quality standard used in Glashütte.
According to an unwritten, however generally regarded as binding convention in the German watch town Glashütte, only that watch may bear "Glashütte" as designation of origin, whose "overwhelming" value share has been generated in Glashütte.
Important conflicts that were brought before the court because of this rule, for example happened between the manufacturers Nomos-Uhr-Gesellschaft, Guido Müller & Co, Glashütte i/S., Nomos and Mühle. (For details, see the links in question.)
For watch buyers it is important not to confuse rewordings like "Glashütter Tradition" or "Montiert in Glashütte" (Mounted in Glashütte) with the unique titling "Glashütte", because they should not be equated with this and are therefore rather meaningless.