Everything about the current selection, in one place.
The Properties panel shows what is selected and lets you change it. Select a markup and it shows
that markup's colour, size and text; select several and it shows what they have in common, so an
edit applies to all of them. Selecting nothing leaves it empty.
Go to View > Panels > Properties to show it.
Notes
It sizes itself to whatever it is showing rather than filling a fixed strip, and docks to the bottom of the window by default, where a wide, short panel suits rows of settings better than a tall narrow one.
Its contents follow the selection, not the module: the same panel serves a markup and a materials takeoff item, showing whichever settings the selection actually has.
Properties can also be opened as a floating window, but only one of the two is ever up. Bringing the panel forward, whether from View > Panels, its tab, or by floating it, closes the window. Two properties views bound to the same selection would each go stale the moment the other was used.
Editing several at once
Gather them first, then edit. Right-clicking a markup on the canvas offers to select everything
drawn with that same tool, and selecting rows in the Markup panel selects
those markups too. Properties then edits the whole batch.