What is abi-blacklist in target specification json?

What does abi-blacklist property in target specification JSON mean? For example, in aarch64-unknown-none.json I see the following ABIs blacklisted:

"abi-blacklist": [

Is it a required property? How can improper or missing values affect produced binary?

these seem to be FFI calling conventions that don't exist or are otherwise not supported for the target architecture
I'd guess specifying these wrong (and if they're used in the code) will usually result in LLVM rejecting the inputt