Is there a version of .NET for which I should compile my ArcFM customizations?
For 10.2.1c and below, we still recommend that all ArcFM Desktop customizations be compiled for .NET 3.5. It may be possible to build customizations at higher .NET levels, and they may work, but we can't guarantee that all of ArcFM will work properly if the customizations are built at the wrong level.
For ArcFM Server, It really depends on the version of ArcGIS Server you are using. In general, we recommend using whatever version of .NET that ESRI recommends for Server-specific customizations. For ArcGIS Server 10.4, that would be 4.5, but for other versions it can vary.