alt is used to describe alternative scenarios of a workflow. Only one of the options will be executed.
opt is used to describe an optional step in the workflow.
For example, for an online shop purchase sequence diagram you may use opt to describe how the user can add gift wrapping if she wishes. alt may be used to describe two variants of payment: using credit card or wire money transfer.
In the UML specification the meaning is described in section 17.12.15. opt and alt are two different operator types and here’s how the specification describes them:
alt:
The InteractionOperatorKind alt designates that the CombinedFragment represents a choice of behavior. At
most one of the operands will be chosen. The chosen operand must have an explicit or implicit guard
expression that evaluates to true at this point in the interaction. An implicit true guard is implied if the operand
has no guard.
opt:
The InteractionOperatorKind opt designates that the CombinedFragment represents a choice of behavior where
either the (sole) operand happens or nothing happens. An option is semantically equivalent to an alternative
CombinedFragment where there is one operand with non-empty content and the second operand is empty.