IS-IS is an IGP, and thus applies only to routes within a single routing domain. However, you can configure multiple areas within a domain. A device can be a member of one area for each Network Entity Title (NET) you configure on the device. The NET contains the area ID for the area the NET is in.
In Intermediate systems and end systems, Routers A, B, and C are in area 1. Routers D and E are in area 2. All the routers are in the same domain.