https://deskthority.net/wiki/Keyboard_profile#NumberingThe ISO/IEC 9995-1 standard mandates use of letters. For the main five rows, it is A (next to space bar) to E (numerical row). Cherry, Topre and Leopold use letters.
There is no standard for using numbers instead of letters - each manufacturer that uses numbers has come up with that on their own.
Because Signature Plastics uses numbers and they have been used for many runs of keycap sets, their numerical row naming has sometimes been used for runs from other manufacturers.Almost no new keyboard uses different profiles for row A or above E any more though. Those exist mostly on vintage keyboards. You will almost always see row B (SP R4) profile used also for the space bar row and row E (SP R1) also for function keys.
BTW. Some manufacturers emboss a number on the inside of the keycap showing which mould/part it is from: e.g. "E12" for twelfth key on row E.
