ForeFlight uses the World Geodetic System 1984 standard (aka: WGS84, EPSG:4326). You can learn more about this standard here:
http://en.wikipedia.org/wiki/World_Geodetic_System
ForeFlight does not use NAD 83 or NAD 27, however, WGS84 is very similar to NAD83. WGS84 and NAD83 are often "paired" together because they are so close and only differ at very close scale.
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