Choose the impedance of the complete signal path
Match the connector, cable and equipment impedance. A connector that physically mates is not necessarily an electrical match.
50 Ω or 75 Ω?
50 Ω is common in RF test equipment and wireless systems. 75 Ω is common in video and broadcast systems. Specify the impedance explicitly; a family name such as BNC or N does not identify it.
What happens at a mismatch?
An impedance change causes reflections and can increase VSWR. A mechanical adapter does not automatically transform impedance. For measurements, select a suitable matching device and account for its loss in calibration.
What should your RFQ include?
State equipment impedance, both mating interfaces, frequency range, cable type and acceptable return loss. Check the exact drawing before mating different impedance variants.