This is the mathematical difference between the repeater's transmit frequency and the receive frequency. Since this is a standard value in most areas of North America, it is commonly referenced simply with the direction of the offset, positive (+) or negative (-).
In addition, a repeater may be simplex, 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.