A Transition represents an arc between exactly one source Vertex and exactly one Target vertex (the source and targets may be the same Vertex). It may form part of a compound transition, which takes the StateMachine from one steady State configuration...

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Transitions
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Unified Modeling Language 2.5.1
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About Object Management Group copyright in text extracts quoted from OMG specifications for educational purposes
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INFO
UML keywords
State
StateMachine
Transition
Vertex
compound transition
Transition::source
Transition::target
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A Transition represents an arc between exactly one source Vertex and exactly one Target vertex (the source and targets may be the same Vertex). It may form part of a compound transition, which takes the StateMachine from one steady State configuration to another, representing the full response of the StateMachine to an occurrence of an Event that triggered it.
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Transition::kind : TransitionKind [1..1] = external Indicates the precise type of the Transition.
Transition::container : Region [1..1] ... Designates the Region that owns this Transition.
Transition::effect : Behavior [0..1] ... Specifies an optional behavior to be performed when the Transition fires.
Transition::guard : Constraint [0..1] ... A guard is a Constraint that provides a fine-grained control over the firing of the Transition. The guard is evaluated when an Event occurrence is dispatched by the StateMachine. ...
Transition::guard : Constraint [0..1] ... If the guard is true at that time, the Transition may be enabled, otherwise, it is disabled. Guards should be pure expressions without side effects. Guard expressions with side effects are ill formed.
Transition::redefinedTransition : Transition [0..1] ... The Transition that is redefined by this Transition.
Transition::/redefinitionContext : Classifier [1..1] ... References the Classifier in which context this element may be redefined.
Transition::source : Vertex [1..1] ... Designates the originating Vertex (State or Pseudostate) of the Transition.
Transition::target : Vertex [1..1] ... Designates the target Vertex that is reached when the Transition is taken.
Transition::trigger : Trigger [0..*] ... Specifies the Triggers that may fire the transition
If the composite State is also an orthogonal State with multiple Regions, each of its Regions is also entered, either by default or explicitly.
If the Transition terminates on the edge of the composite State (i.e., without entering the State), then all the Regions are entered using the default entry rule above.
If the Transition explicitly enters one or more Regions (in case of a fork), these Regions are entered explicitly and the others by default.
Regardless of how a State is entered, the StateMachine is deemed to be “in” that State even before any entry Behavior or effect Behavior (if defined) of that State start executing.
Exit points are the inverse of entry points. That is, Transitions originating from a Vertex within the composite State can terminate on the exit point.
In a well-formed model, such a Transition should have a corresponding external Transition outgoing from the same exit point, representing a continuation of the terminating Transition.
If the composite State has an exit Behavior defined, it is executed after any effect Behavior of the incoming inside Transition and before any effect Behavior of the outgoing external Transition.
Entry points represent termination points (sources) for incoming Transitions and origination points (targets) for Transitions that terminate on some internal Vertex of the composite State.
In effect, the latter is a continuation of the external incoming Transition, with the proviso that the execution of the entry Behavior of the composite State (if defined) occurs between the effect Behavior of the incoming Transition and the effect ...
If there is no outgoing Transition inside the composite State, then the incoming Transition simply performs a default State entry.
A Transition is a single directed arc originating from a single source Vertex and terminating on a single target Vertex (the source and target may be the same Vertex), which specifies a valid fragment of a StateMachine Behavior.
It may have an associated effect Behavior, which is executed when the Transition is traversed (executed).
NOTE. The duration of a Transition traversal is undefined, allowing for different semantic interpretations, including both “zero” and non-“zero” time.
Transitions are executed as part of a more complex compound transition that takes a StateMachine execution from one stable state configuration to another.
A Transition may own a set of Triggers, each of which specifies an Event whose occurrence, when dispatched, may trigger traversal of the Transition. A Transition trigger is said to be enabled if the dispatched Event occurrence matches its Event type.
When multiple triggers are defined for a Transition, they are logically disjunctive, that is, if any of them are enabled, the Transition will be triggered.
The semantics of a Transition depend on its relationship to its source Vertex. Three different possibilities are defined, depending on the value of the Transition’s kind attribute ...
kind = external means that the Transition exits its source Vertex. If the Vertex is a State, then executing this Transition will result in the execution of any associated exit Behavior of that State.
kind = local is the opposite of external, meaning that the Transition does not exit its containing State (and, hence, the exit Behavior of the containing State will not be executed). ...
kind = local ... However, for local Transitions the target Vertex must be different from its source Vertex. A local Transition can only exist within a composite State.
kind = internal is a special case of a local Transition that is a self-transition (i.e., with the same source and target States), such that the State is never exited (and, thus, not re-entered), which means that no exit or entry Behaviors are executed ...
kind = internal ... This kind of Transition can only be defined if the source Vertex is a State.
Transitions of the kind internal are not shown explicitly in diagrams.
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