FLE means Floating Lens Element which the user manually sets based on distance to subject. It specifically optimized optical performance at various distances, but in particular improved closer focus.
The Zeiss 50mm 200 series lens was a non-FLE with a f/2.8 maximum aperture as opposed to f/4 of the 500 series … which was possible because there is no central shutter in the lens.

The latest version of the 40mm is the Zeiss 40CFE IF, which significantly improved corner to corner, edge to edge performance over the previous 40mm but at the expense of some additional distortion … which Phocus automatically corrects with a mouse click … software easily fixes this type of issue compared to edge sharpness issues which it can’t fix. I would consider the 40IF a digital inspired lens since it was developed well after digital had become dominate.

The lens designation C, CF, CFi CFE meanings are: “C” Central Shutter, “F” Focal Plane (meaning there is an F setting on the 500 series V lens that disables the central shutter so the lens can be used on the 200 series Focal Plane cameras) The “i” of CFi lenses means “improved” (newer locking sync port, better reflection suppression inside the lens, improved internal springs, and better hood mount coating, etc.) The E of CFE stands for Electronic data bus contacts which allowed for automatic meter indexing when used on a 200 series camera that has a built-in meter, and also indexes itself when a CFE Zeiss lens is used on an H camera via the CF adapter.

If one is using a CFV/16 back on a V camera, the crop factor is 1.5X, so some of the older lenses are less of an issue compared to using the CFV/39 or 50. If using the CFV 39/50, I strongly suggest the 40IF and 50CFi FLE for optimized optical wide angle performance.