  The development of Autonomous Vehicles (AVs) has made significant progress in
the last years. An essential aspect in the development of AVs is the assessment
of quality and performance aspects of the AVs, such as safety, comfort, and
efficiency. Among other methods, a scenario-based approach has been proposed.
With scenario-based testing, the AV is subjected to a collection of scenarios
that represent real-world situations.
  The collection of scenarios needs to cover the variety of what an AV can
encounter in real traffic. As a result, many different scenarios are
considered, that are grouped into so-called scenario categories. We propose a
method for defining the scenario categories using a system of tags, where each
tag describes a particular characteristic of a scenario category.
  There is a balance between having generic scenario categories - very specific
set of scenarios, while for another system one might be interested in a set of
scenarios with a high variety. To accommodate this, tags are structured in
trees. The different layers of the trees can be regarded as different
abstraction levels.
  Next to presenting the method for describing scenario categories using tags,
we will illustrate the method by showing applicable trees of tags using
concrete examples in the Singapore traffic system. Trees of tags are shown for
the vehicle under test, the dynamic environment (e.g., the other road users),
the static environment (e.g., the road layout), and the environmental
conditions (weather and lighting conditions). Few examples are presented to
illustrate the proposed method for defining the scenario categories using tags.
