Commercial Mailboxes, also known as Cluster Mailboxes or CBUs, are mailboxes with multiple compartments. Each compartment has a number plate on it. The number plate comprises a unique number or address of the owner to which the mail gets delivered. Commercial Mailboxes are designed for multi-tenants or workers in a building. That is why the name is the commercial mailbox. They come in use for multiple people living in a building or area. Unlike the curbside residential mailbox, which has one or two compartments, the commercial mailbox can have more than 16 compartments.
Source: Commercial Mailboxes