Service Scalibility is the flexibility of a billing application to offer a wide range services, not just Asterisk related services, but also ad hoc charges and invoices, and any type of service a company may wish to offer in the present.
Such services could include ISP, WISP, Wi-Fi, Hotspot, Hosting, Time Billing, etc. This is also referred to as "Unified Billing" or "Convergent Billing".
A good billing application for Asterisk or VoIP will have the primary focus on the billing aspect independant of Asterisk. Many billing solutions that have been developed primarily for Asterisk services or for VoIP type services in general will be limited to the point that adding non-Asterisk related services down the road will be impossible or difficult. This is not desirable, as it makes it impossible or costly for a company to roll out new services as they scale.