Full-duplex repeaters receive and transmit at the same time, requiring two separate frequency channels. The offset is the mathematical difference between the repeater's transmit (downlink) frequency and the receive (uplink) frequency. The advent of the band plan has made this a predictable value for most repeaters, and as such can often be referred to simply by the shift, which is positive (+) or negative (-).
Some repeaters or nodes may operate on a single simplex channel, where the transmit and receive frequency is the same. This is noted with an “x”. If a repeater has an odd (non-standard split/offset) then it is noted with an “s”.
For the “Rest of World” repeaters, the offset designators are not used. Repeater data is entered with both the transmit and the receive frequencies. The offset is then calculated and displayed automatically.