If you are looking for sources for F # 3.0 AllowCrossTargeting, it appears only in one place fsharp/src/fsharp/Salsa/salsa.fs, line 686.
Since it seems to be a write property only for F #, I assume that it has something to do with the F # compiler running on one version of the .NET platform, and targeting another version. In addition, a Google search for a property name only calls files .fsproj, so it does not appear. AllowCrossTargetingdoing something right now.