Bacs is a Not-for-Profit organisation that is owned by 16 of the leading banks and building societies in the UK, Europe and US. These are called the Member banks. Member banks have a direct link to Bacs and will upload transactions to Bacs directly.
There are hundreds of other banks (and building societies) that can offer accounts that support Direct Debits however. These are called Agency banks. They are not direct members of Bacs and will themselves bank through one of the Member banks. They will send their transactions to their Member bank and the Member bank will send the transactions to Bacs on their behalf.
Other terms that you might see are Sponsor Banks and Paying banks. Sponsor banks are actually the member banks. They sponsor organisations, and the Agency Banks to use the scheme, i.e. it is their authority that is required for an organisation to be authorised as customers who can use the Bacs scheme.
Paying banks are any banks who offer accounts that will accept Bacs Payments.
All banks, whether Member banks or Agency banks are governed by the same set of rules that regulate the scheme. How they process transactions can be different however and this in turn can have an impact on your organisation. It is worth knowing therefore if your bank is an Agency or Member bank.
An Agency bank is any bank or building society that is not a member of Bacs but is sponsored to participate in the Direct Debit or Direct Credit scheme by a Member bank.