I looked at the parts list, fortunately there are just 2.5" and 3.5" carriers, there aren't any specifically designed for the front or the rear of the chassis. I do have a variety of part numbers, X7K8W is one of those. You should be fine to use it in either location.
I am about to order a Poweredge R540 Server with the 12 x 3.5" bay front + 2 x 3.5" Flex rear bay option. Is anyone able to shed some light on whether the rear flex drive bay accepts the same drive (caddy) or is the caddy different in some way.   Is the caddy different in some way, or can i just take a front drive and plug it into the rear? Reason being is I have to order at least one drive drive for front and one for the rear, but would like to be able to order 1 x 480GB front, and to the rear to make a 2 x 480GB RAID 1 mirrored drive for the OS.  The 12 bays in the front would have 16TB 3rd part drives for backup storage. The Caddy part I have found for the front is: X7K8W, but cannot find anything that relates to a rear "Flex" caddy, or even those drives.
Difference between Front and Rear drive bays in a R540 Poweredge server