Most of the local addresses in the CPU local map always "point" to well-defined fixed physical locations. Certain local addresses, however, will not always point to the same physical locations. These special address ranges are called "page windows". Local addresses inside a page-window range are addresses whose mere 16-bits are not enough information for the MMU to determine a well-defined physical location.
For a local address inside a page window range, the MMU requires additional information, stored inside a register, to be able to translate the given local address into a well-defined physical location. Such a register is called a Page-register.