Contexts correspond to UIMA Pipelines. A context is associated with an Annotator (Primitive or Aggregate), or (for an aggregate, also its flow controller).
There is a "root" context. Aggregates have children and a structure corresponding to the directed acyclical graph of the subparts.
There is a variant / wrapper for context for PEARs, which substitutes a class path associated with the Pear.
There is a variant / subclass for Flow Controller components