18.104.22.168.5 FMap (Free Map) Page
The general layout of an FMap is identical to that of an AMap, except that each byte in the FMap corresponds to one AMap page. The value of each byte indicates the longest number of free bits found in the corresponding AMap page. Generally, because each AMap covers about 250 kilobytes of data, each FMap page (496 bytes) covers around 125 megabytes of data.
Implementations SHOULD NOT use FMaps. The Density List SHOULD be used for location free space.<16> However, the presence of FMap pages at the correct intervals MUST be preserved, and all corresponding checksums MUST be maintained for a PST file to remain valid.