Choice   A choice is a pseudostate that acts a decision point. It has a single incoming transition and two or more outgoing transitions one (and only one) of which will fire as a result of the firing of the incoming transition. Each outgoing transition is guarded by a condition, which must evaluate to "TRUE" or "FALSE". A transition will fire if and only if its guard evaluates to "TRUE".

Explanation

A decision acts as a junction point that determines which one of a number of possible paths will be followed when the decision is reached and the guards (conditions) on the outgoing paths are evaluated. It is important that at least one of the outgoing paths can fire otherwise the execution will stall. It is also important that one and only one of the guards is true.

Important Points

		All outgoing transitions must be guarded.		All outgoing transitions from a decision must be guarded to ensure that there is no ambiguity about which one of the transitions will fire.
		The guards must be non overlapping.		If one guard evaluates to "true", then no other guard can evaluate to true under any conditions. This is tantamount to saying the guards must be non overlapping or mutually exclusive.
		There can be one (and only one) incoming transition to a decision.		There can be one and only one incoming transition to a decision, but there can be two or more outgoing transitions.
		There can be two or more outgoing transitions.		A decision has two or more outgoing transitions. These must be guarded and the guard conditions must be mutually exclusive. These restrictions ensure that in all executions of the decision one (and only one) transition will "fire".

Related Entries

Junction  Used Together,  Paired Element A junction is related to a choice as a paired element. Both the junction and the choice are pseudostates but whereas the choice has a single incoming transition and two or more outgoing transitions, the junction can have any number of incoming transitions and one (or more) outgoing transitions. It can be used to re-unite transitions that have been split by a choice pseudostate.

Fork (Vertical)  Confused with A fork is related to a choice in the sense that both elements have a single incoming transition and a number of outgoing transitions. There are however a number of important differences. Firstly the fork splits an incoming transition into a number of concurrent outgoing transitions which all fire at the same time, whereas with a decision one (and only one) of the outgoing transitions fires. Secondly the outgoing transitions may not normally be guarded (when forks are used in activity diagrams the outgoing transitions can be guarded) whereas the transitions exiting a decision must always be guarded.

N-ary Association  Graphically Similar,  Confused with An n-ary association is related to a decision only because it has the same fundamental shape - the diamond with three or more attached lines. Semantically they are not related. The lines in the n-ary associations are associations representing structural relationships, whereas the incoming and outgoing lines (transitions) in a decision signify changes in state. They also appear in different UML diagrams. The n-ary association is used in the same diagrams where binary associations are found, such as class, component and deployment diagrams, whereas the decision is found in state and activity diagrams.