Business continuity is the overall capability of the organization to keep operating through disruption — people, processes, sites, supply and technology. Digital continuity is the technology layer within that: the systems, data, identity and infrastructure that must keep working.
Digital continuity is a precondition of business continuity. Without recoverable systems, identity and data, the rest of the plan cannot operate.
The two must connect: a business continuity plan that assumes technology "just continues" is incomplete. Digital continuity provides the assurance that assumption is valid.