Edge codes were a practice established by manufactures. In general they don't seem to be government mandated, but something the industry found helpful for themselves and maybe because the OEMs wanted it. However, since I last looked at this two states have mandated a phased reduction of certain lining materials and a revised edge code is being used to show the compliance level of the material.

The marking standard is SAE J866
The first letters represent the manufacturer.
(web search for a AMECA list of edge codes for abreviations)
The next segment is the material.
This is followed by a two letter Friction Code.
The Friction code is based on the SAE J661.

The new marking standard adds onto that as shown.

[img]https://www.toyota-4runner.org/atta...um-brake-shoe-replacement-leaf_codes-jpg[/img]

Not all manufacturer's complied with marking the old friction code when selling aftermarket pads and shoes. But most did.

The J661 specifies a pretty basic test to come up with a coefficient of friction in two temperature ranges:
First letter represents the "Normal" test coefficient of friction (some interpret as 0-250 degrees F)
Second letter represents "Hot" coefficient of friction from (some say representative of 200-600 degrees F)

D Over 0.15 up to 0.25
E Over 0.25 up to 0.35
F Over 0.35 up tp 0.45
G Over 0.45 up to 0.55
H Over 0.55

So for example a shoe marked ED would suggest it looses friction once over 200 or 250 F. On drum brake, especially in the front, that could mean a notible loss in braking when hot and increasing force needed on the pedal just to get maintain the stopping power. Maybe on the rear effect would not as bad and even reduce possibility of lock up.

Keep in mind these ratings only are a rough indication. For example they don't show the trends from 200 to 600 F, and the range of friction coeficient within each letter designation is really wide. They also don't reflect other real application characteristics, just friction of a sample against a turning wheel.

As far as the temperatures go, an article in SHO Times gives some idea of how hot an undersized OEM brake can get. These are heavy cars and the front disks were pretty small.

"...measured 385 F on my front rotors in rush hour traffic on a 45 mph street because of several sequential stoplights. (I have an 89 SHO with 10.1 in. rotors.)
...four back to back 60 mph stops generated 550 F."