Newbie here, can someone explain to me what exactly does this phrase mean?
Nextels and public safety were essentially using adjacent intermixed frequencies. An "agreement" was reached to move public safety exclusively to one portion of the band and Nextel exclusively to another portion so they don't so easily interfere with each other.
While there are typical scenarios of changes taking place, there is no hard fact to a "15 MHz" change. All that matters is that users are separated as I stated.
Unless the agency is using specific frequencies AND a Motorola trunked radio system, all scanner users need to do is program the new frequencies once the agency moves. But if certain frequencies are selected as voice channels AND they are using a MOT type TRS, then you need a scanner that can handle the change to be able to trunk track (have the scanner automatically follow conversations).