UUID of the SII key (digital-certificate credential).

UUID of the company on whose behalf the request is made. Fiscal-invoice endpoints
authorize the caller against this company (and read/write its data). SII-key
endpoints never filter their results by this header. On the SII-key list,
however, belonging to the header company is one of the ways a caller is authorized
to make the call (alongside being Cardda staff or passing their own user_id), so
the header is not merely ignored there. On the other SII-key endpoints the header
plays no part in authorization. See The company-id header
for details on how to obtain a value for this header and the error responses to
expect when it is missing or invalid.