A BIC (also called a SWIFT code) identifies a financial institution. The format is fixed: 4 letters bank, 2 letters ISO country, 2 alphanumeric location, and an optional 3-character branch suffix. An 8-character BIC implies the head office; an 11-character BIC adds a specific branch.
The BIC is administered by SWIFT but most rails today derive routing from the IBAN's national-identifier component, so the BIC is increasingly informational rather than load-bearing. SEPA SCT under the "IBAN-only" rule (in force since 2016) no longer requires a BIC for euro-area domestic transfers.
Where BIC still matters: cross-border CBPR+ MX flows (the <DbtrAgt>/<CdtrAgt> block expects it), correspondent-banking routing via the SWIFT network, and any rail that has not adopted the IBAN-only convention.
iso-compliant accepts BIC fields where the underlying message envelope expects them and treats it as optional where SEPA has dropped the requirement.