UML Actors (aka Roles) are meant to provide a twofold description of interactions between systems and their environment: organization and business process on one hand, system and applications on the other hand.
That can only be achieved by maintaining a conceptual distinction between actual agents, able to physically interact with systems, and actors (aka roles), which are their symbolic avatars as perceived by applications.
As far as the purpose is to describe interactions, actors should be primary characterized by the nature of language (symbolic or not), and identification coupling (biological or managed):
Symbolic communication, no biological identification (systems)
Analog communication, no biological identification (active devices or equipments)
Analog communication, biological identification (live organisms)
While there has been some confusion between actors (or roles) and agents, a clear-cut distinction is now a necessity due to the centrality of privacy issues, whether it is from business or regulatory perspective.
States are used to describe relevant aspects in contexts and how the changes are to affect systems representations and behaviors.
On that account, events and states are complementary: the former are to notify relevant changes, the latter are to represent the partitions (²) that makes these changes relevant. Transitions are used to map the causes and effects of changes.
State of physical objects.
State of processes’ execution.
State of actors’ expectations.
State of symbolic representations.
Beside alignment with events, defining states consistently across objects, processes, and actors is to significantly enhance the traceability and transparency of architectures designs.
Depending on context and purpose requirements can be understood as customary documents, contents, or work in progress.
Given that requirements are the entry point of engineering processes, nothing should be assumed regarding their form (natural, formal, or specific language), or content (business, functional, non-functional, or technical).
Depending on the language used, requirements can be directly analyzed and engineered, or may have to be first formatted (aka captured).
Requirements taxonomy should be set with regard to processes (business or architecture driven) and stakeholders (business units or enterprise architecture).
Depending on content and context, requirements can be engineered as a whole (agile development models), or set apart as to tally with external dependencies (phased development models).