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The Solidity documentation has the following:
Internally, Solidity performs a revert operation (instruction
0xfd) for a require-style exception and executes an invalid operation (instruction0xfe) to throw an assert-style exception.
When looking in the yellow paper, I cannot see the opcodes 0xfd and 0xfe documented. (The largest non-suicide opcode is 0xf4 for DELEGATECALL.) Where are 0xfd and 0xfe formally documented?
This is probably a result of the technology moving faster than updates to the yellow paper. Your best bet is to go to the source. Pyethereum is a good source for readability. You can see the latest opcodes here (where 0xfd) is referenced. I would also look here for more details and a potential hint into invalid operations. EIPs such as the one for revert are also a good source of information: REVERT
