Consider the highlighted phrases in the examples below.
- The rice is marvellous. [S1A-022 #262]
- He was a really nice guy. [S1A-006 #21]
Each of the highlighted phrases adds information about the person or thing picked out by the Subject. Marvellous attributes a property to the rice, and a really nice guy does the same for he.
These phrases have different forms but the same function.