This is a partial excerpt from help on the Format event:
"There are times when Microsoft Access must return to previous sections on a report to perform multiple formatting passes. When this happens, the Retreat event occurs as the report returns to each previous section, and the Format event occurs more than once for each section. You can run a macro or event procedure when the Retreat event occurs to undo any changes that you made when the Format event occurred for the section. This is useful when your Format macro or event procedure carries out actions, such as calculating page totals or controlling the size of a section, that you want to perform only once for each section."
And more help:
"Microsoft Access increments the FormatCount property each time the OnFormat property setting is evaluated for the current section. As the next section is formatted, Microsoft Access resets the FormatCount property to 1.
Under some circumstances, Microsoft Access formats a section more than once. For example, you might design a report in which the KeepTogether property for the detail section is set to Yes. When Microsoft Access reaches the bottom of a page, it formats the current detail section once to see if it will fit. If it doesn't fit, Microsoft Access moves to the next page and formats the detail section again. In this case, the setting for the FormatCount property for the detail section is 2 because it was formatted twice before it was printed."