State Transition Graphs
A State Transition Graph (STG) is a directed graph representing the dynamical behaviour of a. Nodes of this graph represent possible states of the model, assigning a value to each component. Arcs of the STG represent transitions from one state to another (i.e. change of value for one or several components).
For a more formal definition see Naldi 2011.
Hierarchical Transition Graphs
The Hierarchical Transition Graphs (HTG) is an acyclic graph, which provides a compact representation of the State Transition Graph. It improving on the. by merging linear chains of states (in addition to cycles) into single nodes.
More information on this graph is available in Berenguier et al (2013).