A check digit is a digit computed from the rest of an identifier so that a mistyped one usually fails arithmetic before anybody looks it up. It catches typos and transpositions; it proves nothing about whether the identifier was ever issued.
Where Spaw computes one
Two places, and both say the same thing about what it means.
checksum_valid on a business answer is the ISO 7064 MOD 97-10 check over a LEI. It is reported separately from the register lookup precisely so the two are never confused: a well-formed LEI that no register carries is a well-formed LEI that no register carries.
The card number checker computes the Luhn digit over a card number entirely in the visitor's browser — no request, no endpoint, no log line, and the number never appears in a URL.
Why the distinction is worth a page
"Valid" is the most overloaded word in identifier checking. A check digit answers one narrow question: were these characters plausibly copied correctly. Whether the thing exists, whether it is current, and whether it belongs to who you were told, are three further questions with three further answers — and this product keeps them in separate fields rather than collapsing them into one boolean.