The purpose of the motor frame system is to allow you to substitute one manufacturer's motor for another brand, if it's not a custom OEM frame. In order for a motor to list a specific motor frame on it's nameplate, it must have mounting dimensions common to that frame. Notice I said mounting dimensions. That doesn't mean every 184T frame motor is exactly alike, just that they will mount in the same location as other 184T frames. So, what are the mounting dimensions?
The most common mounting dimensions are as follows (the dimension designations are in parentheses - NEMA first, then IEC):
Shaft height (D), (U)
Shaft length (N-W), (E)
Shaft diameter (U), (D)
Distance from front mounting to back of shaft (BA), (C)
Front-to-back mounting holes (2F), (B)
Side-to-side mounting holes (2E), (A)
C or D flange mounting hole diameter (AJ), (M)
C or D flange mounting lip (AK), (N)
Let's discuss NEMA frames first. NEMA frames are grouped into families in which all the mounting dimensions are the same except the 2F dimension. Dimensions other than the mounting dimensions can be different. This is most easily seen in the "C" dimension, which is the distance from the tip of the shaft to the back of the motor. Since it's not a mounting dimension, there are no standards from one brand to another. Today's frames are "T" frames. From 1952-1964, the "U" frames were standard. The "original" frames were built prior to 1952.
Now, notice I put shaft height first. There's a good reason for that. If you take the first two digits of a NEMA "T" frame and divide it by four, you will get the shaft height. For example, on a 184T frame, that would be 18 divided by 4, or 4.5 inches. Conversely, if you measure the shaft height of a NEMA T frame motor and multiply by 4, you'll have the first two digits of the frame size.
Let's say you have a motor where you can't read parts of the nameplate. If you measure the shaft height as 4.5" and multiply by 4, you know you have a 182T or 184T frame motor. You have just eliminated all the other possible frames with one quick measurement. Now, all you have to do is check the 2F dimension to see which of the two frames you have. If you know any two out of the horsepower, frame size, and RPM, you can usually figure out the third. So, in our motor with the 4.5" shaft height, if we know its speed is 1750 RPM, it's typically a 5 horsepower motor. This is no guarantee because sometimes OEMs don't follow the rules when having a manufacturer build their motors. Also, in the larger frames, there can be frame size differences between an open (ODP) motor and an enclosed (TEFC) motor. If you find a frame size where the last letter is a Z, that indicates a special frame, generally only available from the OEM that had the motor built for them.
The IEC, or metric, motors usually make it a little easier. First of all, the shaft height in millimeters is the frame size. So a 90mm shaft height is a 90 frame. In many cases, the is a letter afterward, like a 90S or 90L, which distinguishes between frames in the 90 family. Again, this will show up in the B (like the NEMA 2F) dimension. The other nice thing is that all dimensions are in whole millimeters. There are no standard motors with a 90.5mm shaft height. The hard part about the IEC motors is they have no shame about not following these standards when they don't want to; and they don't tell you when this happens, like with a NEMA Z designation. This occurs quite regularly with their flanges. A NEMA C flange is designated a B14 flange in the IEC world. A NEMA D flange is called a B5 flange. You pretty much need a metric ruler with you to make sure you know what IEC motor you have.
This is a start in understanding motor frames. Most all manufacturers put dimensional sheets in their catalogs and/or on the internet. Take a look and this will make more sense.