188.8.131.52.6 FPMap (Free Page Map) Page
The general layout of an FPMap is identical to that of an AMap, except that each bit in the FPMap corresponds to a PMap page, and the value of the bit indicates whether there are any free pages within that PMap page. With each PMap covering about 2 megabytes and an FPMap page at 496 bytes, an FPMap page covers about 8 gigabytes of space.
Implementations SHOULD NOT use FPMaps. The Density List SHOULD be used for location free space.<16> However, the presence of FPMap pages at the correct intervals MUST be preserved, and all corresponding checksums MUST be maintained for a PST file to remain valid.