A fit factor is the result of a fit test and only relates to a specific facepiece/wearer combination.
The Assigned Protection Factor (APF) for a specific type and class of RPE is published in BS4275.6 It relates to the likely performance of the whole device when worn correctly and used in accordance with the manufacturer’s instruction (which includes the need for a satisfactory fit testing). When selecting an adequate and suitable RPE, for use at work, the assigned protection factor should be used. For more details consult HSG53.