The short definition
A software company builds and runs the product: hosting, security, updates, bank statement parsing, VAT logic. The accounting firm puts its name, logo, colours and sign-in page on top and hands it to clients as part of its service. The client never needs to know which engine sits underneath.
This is different from simply recommending an off-the-shelf package. With a recommendation, the client signs up with the vendor, sees the vendor’s brand every day and can take their books to any other adviser who uses the same package. With white-label software, the firm is the brand the client sees on every invoice they send and every report they open.