A section is an addressable regional unit of your score.
By design there are no restrictions on sections size. A section can hold an entire movement (e.g., the Allegro part of a sonata) or a single measure. However, sections can be used by generators as output targets, which makes the shorter ones preferable because they provide more granular control.
In the Editor you can only set the section Name, which shows above the top staff in your score. A unique name is required for every section in your score.
Note: the complete list of pending bugs, features and epics is available in Jira.