Retail M4 (or M2) comprises: The M4 private sector's holdings of sterling notes and coin; and sterling denominated 'retail' deposits with UK MFIs. As noted in the August 1992 Quarterly Bulletin (page 317), the definition of the deposits which, along with notes and coin, previously comprised M2 was altered with effect from the flow for December 1992. Retail deposits are defined as deposits which arise from a customer's acceptance of an advertised rate (including nil) for a particular product, typically 'retail' deposits are taken in the reporters' branch networks. With effect from October 2007, there was a change in the treatment of non-interest bearing deposits (see Valuation and Breaks). Acceptances The treatment of bank acceptances changed in September 1997, and was backdated. Acceptances are still being reported off balance sheet by individual reporters, but in the aggregated accounts and monetary statistics they count as on balance sheet of the accepting reporters (i.e. reporters accepting a bill are regarded as having a liability to the bill's owner and a claim on the party whose bill has been accepted). Other MFI deposits include all Tax Exempt Special Savings Accounts (TESSAs) (until maturity in April 2004) and (cash) Individual Savings Accounts (ISAs). Repos include sale and repurchase agreements against marketable securities of all kinds. An institution selling a security under a repo retains the security on its balance sheet and records a liability to the counterparty representing the cash received. Thus, a repo is a kind of secured deposit. The matching asset will depend on how the cash proceeds were employed. An institution acquiring a security under a reverse repo does not include the security on its balance sheet but records a claim on the counterparty representing the funds lent. The matching liability will depend on how the loan is funded.

Source: https://www.bankofengland.co.uk/statistics/details/further-details-about-m4-data