JEDEC seems to number them according to one less than the number of terminals, thus TRIACs, and thyristors, and darlington transiostors get 2N numbers even though they contain 3 or more junctions.
optocouplers come in with numbers like 3N 4N 5N 6N again based on pinout more than internals.
Bridge rectifiers are 1N or 3N seemingly at random, it's possible that one or the other is not a JEDEC part number.
JIS seems to do the same as JEDEC, although I have not encountered any JDEC parts other than diodes and transistors (and darlington transistors)
Pro-Elecron (EECA) is completely different.
In the second digit I have seen only diodes with the letter N.Does it represent the material the diode is made of and what other letters can we have instead of N?
All JEDEC part numbers have N second
All JIS numbers have S second
so far as I know only Pro-Electron codes the semiconductor material type in the part number. it's the first letter
A for germanium,
B for silicon,
C for alloy semiconductors like gallium arsenide or tungsten carbide
There are lots of parts that are not numbered by any of these schemes or only use a numbering inspired by one of them. in fact is common for part numbers that rhyme to be similar, for example 2N3055 TIP3055 and MJE3055 are all the same kind of thing (in different packages)