The horizontal line before and after the branch is a single line.

In the case of alternative branches, the branch lines have to begin and end with transitions. The branch lines can contain more branches.

If the step before the branch is active, then CODESYS processes the first transition of each alternative branch line from left to right. For the first transition that yields ‎TRUE‎, the associated branch line "opens", meaning the step following the transition is activated.

Note that you can click “Parallel” to convert an alternative branch into a parallel branch.