InstrId is the second 35-character reference field on a pain.001 transaction. Where EndToEndId is meant to travel end-to-end, InstrId is more often used as the debtor bank's internal handle — the reference the customer uses to ask the bank "what happened to that one?" rather than the reference the beneficiary will see.
In camt.053 inbound reconciliation InstrId is the canonical fallback when EndToEndId is missing. Some banks (notably HSBC, BNP Paribas on certain channels) make InstrId the *primary* durable carrier in their camt.053 echo — see apps/api/src/lib/bank-recon-handlers/ for per-bank handler decisions.
A pragmatic strategy: set both fields. EndToEndId carries the customer's invoice id; InstrId carries a derived hash or the same value. The matching engine in apps/api/src/lib/reconcile.ts will consider both candidates against the expected-payments list.