This is the information copied from an e-mail that I received from the FD complaint department:

A stopover is a planned break of more than 24 hours on an international trip. One FREE stopover per route is permitted to members per international round trip award.

Conditions
1. Free stopovers are not permitted within a region
2. Free stopovers are only allowed in an international gateway city
3. Only ONE free stopover is allowed per journey
4. Additional stopovers are allowed, but these should be charged

Gateway cities:
BOS, JFK, LGA, EWR, IAH, DTW, ORD, MDW, SEA, SFO, LAX, MEX, ATL, MEM, MIA, HNL, KUL, BKK, NRT, HND, HKG, SIN, NBO, FRA, CDG, ORY, LHR, LGW, STN, AMS, BOM, DEL, MSP

I hope that that clears things up for you.