In order to maintain both pressure and RF sealing, a choke flange and a cover flange should always be mated - NEVER mate two choke flanges. The choke flange has an 'O' ring groove, permitting the use of an 'O' ring to create a pressure tight union.
There are two CPR flange types, CPRF, which is flat-faced, and CPRG, which is grooved. These flanges serve in teh same capacity as cover and choke flanges in regard to pressure and RF sealing. The CPRG flange will accept pressure seals, RF seals, and a third type of seal which is a combination RF and pressure seal.
Any system that uses high power should be pressurized and sealed to prevent RF leakage. Pressurization is important to prevent arcing in the wavesuide. Flanges for WR-3 through WR-22 (round type), cannot be pressurized without special arrangements being considered.
The CMR flange is a non-pressurable, contact-type flange, and should be used only where pressure is not a factor.
These tips are not a complete guide about flange usage, but they can
serve as useful general information.