What it is
The Editor is a dynamic form that provides read-write access to properties of the currently selected element. It is dynamic because its fields update to match the current node type (see the project hierarchy for a discussion on node types).
How does it look/behave
Describing the appearance of the Editor is challenging due to its dynamic nature. The Editor matches the target being edited: for instance, with a note selected, there will be pitch-related fields, while with a part selected, a list of available instruments will show. Detailing every possible field is the same as going into each editable property of every node type, and has already been addressed in the project hierarchy section.
All the fields in the Editor are live controls: every change performed in the Editor takes place immediately, or after a short delay (there is no Save button). The Undo command can be used to roll back any accidental edit. Most of the controls in the Editor are standard widgets the user should be familiar with from the web (check boxes, radio buttons, select boxes, sliders, etc.) but there is also the notable exception of the Picker component, covered separately.
What to Use it For
You use the Editor to alter the intrinsic characteristics of the symbolic elements that make up your music (notes, voices, measures, parts, etc). You need to use it for all editing except deleting, adding or moving elements around.
Note: the complete list of pending bugs, features and epics is available in Jira.