Additional resources:
http://www.sizecoding.org/wiki/DOS
A nice PDF with similar content:
https://pnx.tf/files/x86_opcode_structure_and_instruction_ov...
Essentially, the uppercase letter of an operand is a combination of the operand type (immediate, register, memory) along with how that is encoded (as ModR/M bytes have a register and a register/memory field), while the lowercase letter is the size of the operand (largely 8-bit/16-bit/32-bit/64-bit for the 1-byte opcodes).
It's more or less the same information you get from the intel manuals (specifically appendix 2A of https://www.intel.com/content/www/us/en/developer/articles/t...). There you can also see what e.g. "Jb" means (a byte sized immediate following the instruction that specifies a sign-extended relative offset to the instruction).
One-byte opcodes here differs from 2 byte opcodes (386+ IIRC) prefixed by a 0F byte and even more convoluted stuff added later.
I downvote people when they say they don't know what something is when they could have used a LLM to explain it to them.