The sketch tool allows you to draw on your model - a great way to quickly mark-up issues or proposed changes.
To start sketching:
Open your VR menu by click on "Create".
Click the "Start Sketching" button.
Move your pointer across the model while holding down the trigger to sketch on surfaces.
You can undo/redo your latest sketch strokes by pressing the "Undo" and "Redo" buttons, or clear all your own sketches by pressing "Clear All".
The sketch menu also gives you several options, like setting the sketch color or stroke size.
By un-checking "Sketch on surface", you can sketch in mid-air. This is useful when marking up objects close to you, or when sketching from Dollhouse Mode.