Use is classified by characteristics of authenticated requestors, in two broad groups:
Human users who complete business operations using the system, as Named users.
Processing performed through service calls, plus processing performed by Web users (typically through the Directed Web access feature), classified as Invocation users.
Similarly, invocations are divided into Web users — who use Directed Web access or access PRPC only through services such as portlets — and system-based users. Each completed interaction or service request that performs processing work (as defined) counts as an invocation. Queries and read-only processing typically do not count as invocations.
Some more help on Service Invocation mentions that this classification depends on Pegasystems license -
The precise definition of service invocation for your system depends on your Pegasystems license and the Data-Admin-License instance identified in the System instance, and may not match the definition provided here. For official documentation, contact Global Customer Support.