Neither locomotive builder has been completely systematic and consistent in its nomenclatural conventions: forgiveably so, I think, since technological changes in the ninety years of diesel electric locomotive development (remember, GE built diesel electrics in 1918!) have been so extensive that it is hard to imagine someone at the begining of the history designing a convention that would still make sense today!
EMD has, notoriously, re-interpreted some of its letters: "SW" originaally meant 600 hp with a welded frame, but came to be generic for end-cab switchers of horsepower up to 1500 hp. In the case at hand, "F" almost certainly was originally (1940?) chosen for "freight," with "FP" a decade or so later for lengthened F-style units with steam boilers: it seems likely (though I don't have documentation) that the "P" was because the units could be used on passenger trains. Another two decades (almost: 1968) and-- the original "FP" designs being long out of production-- they recycled "FP" for cowl-body units with steam generators, and then "F" for freight-only SD types with cowls. Amtrak's 1973 "SDP40F" continued this use of "F" for cowl ("full-width"?) bodies; it would probably have been called an "FP40" except for a temporary legal requirement that it be deemed a modified old rather than a new design. When Metra bought similarly cowled SD units with HEP for Chicago commuter service in 1974, they were called "F40C": your guess as to whether "C" meant commuter or Chicago or something else.
GE's sins against nomenclatural consistency are different. They haven't even tried to have a distinct model designation for each design, and have happily applied the same designation to different designs. (This is more obvious when you look at their export locomotives in addition to the North American models: "U30C," for example, can be a North American unit from the 1960s/1970s with a 16-cylinder engine and 752 traction motors OR an export model from the 1980s/1990s with a 12-cylinder engine and 761 traction motors!)
One does have the feeling that the locomotive builders may not care quite as much as the railfans about model designations....