E for essential, do you have a reference? And, S now stands for?
(Do know, I've been retired some years now from networking, and I don't much concern myself with product ordering codes.)
Yea, Cisco certainly can rename, so if E is for essential, S is for subset (of essential)? ; )
I recall, years ago, when there were multiple features images, for routers, each with its own .bin, the Enterprise image, which had all possible features, had a letter K in the .bin file name, which I recall was sometimes referred to as standing for "kitchen sink". Don't recall its ordering product code, either either stand alone nor on a newly purchased router.
Besides naming, Cisco can change what features go into a feature set over the years. Features might vary between product series too. In the past, perhaps, Cisco's Feature Navigator was more important.
One of the most significant that I encountered, was as noted in a prior reply, initially Cisco had both a IP Services and Advanced IP Services for 3750, but they later merged the latter into the former. (Which was a nice cost savings if you needed the latter's features. I don't, though, recall getting a refund for the latter's additional cost if already acquired.)
