What distinguishes a 'multi-tenant' SaaS architecture from a 'single-tenant' architecture?

In a multi-tenant architecture, a single running instance of the application (and typically the underlying infrastructure and, often, database) serves multiple customers ('tenants') simultaneously, with logical isolation ensuring each tenant's data remains separate and secure — this is generally more cost-efficient to operate and maintain, since infrastructure and updates are shared. A single-tenant architecture instead provisions a completely separate, dedicated instance of the application for each customer, offering stronger isolation but typically at higher operational cost and complexity, not lower cost as option B claims. Multi-tenant systems don't require customers to manage their own servers (that would contradict the SaaS model entirely), and single-tenant systems can absolutely be hosted in the cloud, just with dedicated rather than shared resources.