The following mechanical drawing is for a pressure sensor and recommends a 19mm hole with a 20 degree phase for mounting. What does a 20 degree phase mean?
At this point I think, it is just a chamfer at the entrance of the hole that you insert that sensor into to prevent the O-ring to be sheared off as you insert it.
Just a 20 deg chamfer (or bevel) at the start of the 19H8 required mounting hole is recommended for ease of assembly. Just break the corner on the mounting hole and make sure there is no burr.
Using the word "Phase" instead of "Fase" is a translating error. The word "Phase" can have many different meanings in German. Anywhere from a period of time to electricity. That is why one should be very careful when using computer translations for technical terminology. The will kill you. In this case the correct word "Fase" -chamfer or bevel is pretty obvious.
The axial location and radial dimension of the 20deg (per side) chamfer are important, but no definition is provided in the sketch. The radial dimension of the chamfer needs to be at least large enough to guide the free state o-ring during installation without causing damage. The axial location of the chamfer should ensure the o-ring never passes beyond its inner edge under all operating conditions, which could cause a leak.