In my opinion, the verbiage in the code is a bit messed up. When you look at the definitions in article 550, you see mobile home and manufactured home.
The difference is that a manufactured home is morecthan 8 feet wide and 40 fert long, or bigger than 320 square feet.
In other words, what the code calls a mobile home, real people call a travel trailer.
And what the code calls a manufactured home, real people call it a mobile home.
Either way, they're both assembled onto a permanent chassis - wheels and all
A modular home is a real home that's built in sections, and put onto a permanent foundation.
Travel trailers in code are covered in 2017 NEC section 551. RECREATIONAL VEHICLE AND RECREATIONAL VEHICLE PARK
As to your differentiation between manufactured home and a modular home, the only real difference is the lack of a perminant steel structural frame. They both are covered in Article 550. And exempted from a lot of requirements that a stick built house would have to comply with.
This is a factor in manufactured houses of all types, "mobile home", "double wide", or "modular" they all are exempted from many of the building codes that would effect a "stick built" house. If you are building a stick built you are held to a much higher standard both in construction, mechanicals, as well as electrical installation than are "manufactured" homes.
I've worked for a while doing installation of both "modular" and "double wides" not involved with the electrical but did take notice of the subpar installation (from a stick built perspective), both were notorious for mis matched connecting points, both electrically, plumbing, and heating in addition to even the physical dimensions. Worst I've seen was a "mis sized" by over 3 inches, made it interesting to get the siding installed on gable ends. They build to a minimum HUD standards and really stick to doing that to the most minimum that they can "get away" with. Electrically I've seen many with "flying splices" when the sections didn't quite line up, they most times used "boxless splices" (a specialized connector allowed in a manufactured home) but not always. And for those who hate AFCI's, get a manufactured home, they don't use them, exempt, even though NEC 2017 550.25(B) calls for it.
https://www.ecfr.gov/cgi-bin/retrie...n=pt24.5.3280&r=PART&ty=HTML#se24.5.3280_1801