The product defines activity diagrams as meaningful only if join and fork nodes are well-structured in the same sense as well-structured parentheses. In other words, they must use proper nesting. The only exception to this rule is that a join or fork node with multiple ingoing/outgoing control flows can be used in place of a number of join or fork nodes with only two ingoing or outgoing control flows each.
The product tolerates less-than-meaningful activity charts if they can be extended into meaningful ones by adding control flows (for example, a fork with control flows that never merge back).
As you draw activity diagrams, the product prevents you from drawing constructs that violate the meaningfulness of the activity diagram by displaying a "no entry" symbol.