[fmajid]

Sales engineers are pre-sale, FDE are both pre- and post-sale. FDEs are also much more involved in the client implementation, often they are the ones actually doing all the work, specially in government bureaucracies or large ossified corporations where no one there actually knows how to get things done.

[slowin]

Sales engineers do post-sales work. Typically if they're going to do a significant amount of real work, they'd just be called "consultants".