SPEI
SPEI is Banco de México's real-time interbank transfer system for Mexican pesos.
SPEI (Sistema de Pagos Electrónicos Interbancarios) is the real-time interbank payment system operated by Banco de México, Mexico’s central bank. It settles peso transfers between Mexican banks in seconds, 24 hours a day, every day of the year, with no central-bank fee to end users. To send one you need the recipient’s 18-digit CLABE, their name, and an amount in pesos; the sending bank can then issue a CEP, the official electronic receipt. SPEI is required infrastructure for every regulated Mexican bank, which is why a transfer from any Mexican bank to any other works the same way. It moves Mexican pesos only — it does not touch US dollars or cross borders.
See also: What is SPEI? for the long-form explainer.