The AbcCoreAbstract library is an almost-pure abstract library that describes the interfaces that the storage layer (e.g. AbcCoreHDF5) has to implement for Alembic. The AbcCoreAbstract layer specifies the interface for concepts like Objects or Properties, and the byte-size of data. More on the Alembic concepts like Objects or Properties.
One important piece of Alembic that is defined in this layer is Alembic’s notion of time, in the form of the TimeSampling and TimeSamplingType classes. These are non-abstract classes, and provide a rich interface for working with the temporal component of data samples, and are some of the only classes from this layer that are directly and prominently exposed in higher layers.
AbcCoreAbstract is not intended to be the primary human-friendly data-manipulation library in Alembic; that distinction is held by the Abc library.