The Property Inspector
The Property Inspector allows you to set some common properties for controls, such as alignment, position and size as well as specific settings for certain controls (selection state, text properties, color, etc.). It is available when viewing wireframes, assets, and Symbols.
You can show and hide the Property Inspector by clicking the toggle icon above it (keyboard shortcut CTRL/⌘+; or ALT/OPTION+ENTER). When hidden you can also show it by right-clicking a control and selecting "Properties...".
If you have multiple controls selected it will show properties that are common to all selected controls as well as options for aligning and distributing.
When no controls are selected the Property Inspector shows notes for the selected wireframes, asset, or Symbol. In the wireframes view the panel also shows wireframe alternates.
Note: You can format notes text using the syntax described here to make text bold, underlined, etc.
For assets and Symbols it also shows the count and name of wireframes where the selected object is used.