The peptide ligand binds at the highly conserved cleft formed at the interface between the two tandem BRCT subdomains.
The "aaa" part differentiates parts of the domain, which are called subdomains.
Subdomains can be smaller divisions within a larger website, for example "business.lovetoknow.com" versus insurance.lovetoknow.com.
However a domain covers more than just the website and other subdomains might handle email, file transfer, or other Internet services.
Internet domain registration applies only to the main domain not the subdomains, which are handled by the web host.