Thought I had a post on this but could not locate.
I've seen something where it say RMC can be threaded or unthreaded but then came across this which says it is always threaded.
So my interpretation is when they say RMC that is definitely not EMT.
https://steeltubeinstitute.org/steel-conduit/types-of-steel-conduit/rigid-metal-conduit-rmc/
Rank-ordering the three types of non-flexible metal raceways by thickness:
Rigid Metal Conduit (RMC) = thickest. May also be known as rigid galvanised steel (RGS) or galvanized rigid conduit (GRC).
Intermediate metal conduit (IMC) = intermediate thickness
Electrical metal tubing (EMT) = thinnest
RMC and IMC are very closely related. Both made of threaded ends and coupled with threaded fittings, and can involve threadless fittings if applicable. The difference is that IMC has thinner walls, and is made from a stronger alloy of steel to compensate. They are both considered equally strong and generally can be used in the same applications. IMC has the advantage of being cheaper, lighter in weight, and having slightly more fill area in the same trade size, while RMC is generally more available and easier to field work (threading, bending, etc).
EMT is technically not a conduit, but a tubing. You may find everyone will still call it "conduit" in spite of the semantics. EMT may also be called thinwall. The overall diameter is slightly smaller than its rigid counterpart. EMT cannot be threaded, and is built up of couplings and connectors that either attach by setscrews or compression glands. EMT is commonly used as an interior raceway, in contrast to its rigid counterpart. EMT is not as robust against physical damage, as RMC and IMC, and is therefore not to be used where an inspector would say it is "subject to physical damage".
