Ah, good ol' OP_CODES! They're not exactly forms of digital signatures but let me explain.

OP_CODES are part of Bitcoin's scripting language. You can think of them like the building blocks that make up Bitcoin transactions. Each OP_CODE represents a certain operation in the script - for example, OP_CHECKSIG is used to verify a signature against a public key.

So while they're not types or forms of digital signatures themselves, some OP_CODES do involve working with signatures as part of their function. It's all pretty nifty when you get down into the nuts and bolts!