For a Decision shape, a single rule (must be decision table, decision tree, or map value) is used to compute a value such as 1, 2 or 3. Then, flow execution advances along a connector flow action (arrow) that matches the value.
For a Fork shape, the tests are on the connectors themselves; the tests return true or false. Connector tests are evaluated one by one, based on the likelihood percentages. Flow execution advances along whichever connector first evaluates to true.
(Typically, the tests on the connectors are When rules.)