A piggyback domain is an additional domain added to your system which replicates for all of your members. For example, if the domain name for your system is yoursystem.com, you can also have additional domains setup with replicating pages. These additional domains are called piggyback domains. So, if a user with the username "demo" belongs to yoursystem.com, and you add a piggyback domain called yourpiggyback.com, then yourpiggyback.com/demo will be username demo's replicated page on the piggyback domain.
