Se' means "(radio) frequency" in general. A signal may be received (or sent) on a particular frequency, and what that frequency is is specified in terms of chaDvay'(mey).

When a signal is sent out on a number of frequencies simultaneously, it's said to be sent out on Se'(mey), not chaDvay'(mey). Similarly, when monitoring a number of frequencies, it is Se'(mey) that are being monitored. (Of course, specific chaDvay'-(mey) can be specified when relevant).