One of the most important features of Structure is its ability to visualize large amounts of issues in the hierarchical form based on the relationships which exist between the issues, the field values and other criteria: for example, automatically place stories under their epics, and sub-tasks under their parents, or group issues by Sprints, Fix Versions or assignees.
There are two mechanisms for building such structures: Automation and Synchronizers.
Synchronizers were the original mechanism for building dynamic structures, which was replaced by Automation in Structure 3.0 and is gradually faded out. Starting with version 5.0, synchronizers functionality is not visible anymore if there were no synchronizers installed on the instance previously. If they are already in use, we recommend to gradually "migrate" all structures to use Automation until Synchronizers are not used at all.
Automation is the preferred option and only rarely Synchronizers could be a better fit.