What does the "MID" stand for in engine description?

I am looking over a spreadsheet from Fiat Chrysler Automobiles at RAM 3500 Tradesman. There is a description 6.4L V8 MID DUTY HEMI MDS ENGINE.

What does the "MID" stand for?

This is only speculation about what FCA/Stellantis means but in commercial trucking mid-duty or medium-duty is a classification of truck that's more than a grocery getter pickup truck (class 1 & 2, everything from a Tacoma to a 2500) and less than a 18 wheeler (class 8, sometimes 7) based on GVWR

FCA/Stellantis offers the 6.4 Hemi in a passenger car + SUV trim and in a medium duty truck trim, they have different block castings, different heads, different intake manifolds, different cam - the car + SUV version is built to rev higher and make a higher peak HP while the truck version is built to maintain more consistent temperature under a constant load and deliver better fuel efficiency (to the extent possible) under load with a lower peak horsepower number. I assume that the literature describing their medium duty trucks also identifies that they have this version of the engine in it, to help avoid confusion if someone compares it with the SUV 6.4's HP numbers.

This article describes the differences https://www.dodgegarage.com/news/ar...emi-engine-quick-reference-guide-part-iv.html

In your writing, are you stating (in a commercial truck setting), that "mid-duty" is synonymous with "medium-duty"?

That is the assumption I am making, yes.

See the 4th Gen section in this article:
https://carbuzz.com/cars/ram/3500-generations
It would be interesting to know what the differences are between 6.4L Heavy Duty V8, 6.4L Mid Duty V8, and the 6.4L V8 that was available on the Laramie and Laramie Longhorn.

More reading - https://www.ramforum.com/threads/6-4-swap.128175/page-4

Valves are also differant,between the mid duty and the hd 6.4.The heavy duty 6.4 use sodium filled exhaust valves for better valve and valve guide cooling.

According to Modern Muscle Xtreme's Byron Walker, the BGE has solid-stem intake valves, not hollow ones, and though the exhaust valves on Ram HD (Chassis Cab) 3500, 4500, and 5500 series are sodium-filled, they are not the race-quality units found in the Hellcat. The BGE's sodium-filled exhaust valves—where equipped—can be distinguished by a dark DLC coating on their valve faces and stems. Our BGE 392 was from a 2500 series and has the standard non-coated Apache 392 valves (2.13-/1.65-inch diameter). Stock port flow in untouched form approaches 350 cfm at .600-inch valve lift.