In other words, "D is a B" (B subsumes D, or D is-subsumed-by B) usually means that concept D is a specialization of concept B, and concept B is a generalization of concept D. For instance, a "fruit" is a generalization of "apple", "orange", "mango" and many others. One can say that an apple is a fruit.
In object-oriented programming the is-a relationship arises in the context of inheritance concept. One can say that "apple" may inherit all the properties common to all fruits, such as being a fleshy container for the seed of a plant.
When designing a model (e.g., a computer program) of the real-world relationship between an object and its subordinate, a common error is confusing the relations has-a and is-a.