Boxes
In Marvin JS, you have the ability to draw different graphical objects such as rectangle or ellipse. To draw a shape you can use the Graphical Object tool on the Tools toolbar. This is a "combo " button, you can choose the appropriate shape by left-clicking on the black triangle on the lower right corner of the button.
You can use these shapes as frames of your structures. To distinguish the frames the color of the background and the line is changeable by opening the graphical objects pop-up menu.
There is an opportunity to resize your structure: Left-click on one of the control point of the rectangle/ellipse, and without releasing the mouse button drag the line to the right place.
Snapped objects
When you want to highlight the steps of mechanism by these frames, you have the ability to assign arrows to the control point of the shapes.
You can do it by creation or by dragging the end of the arrow. In this case when you drag the shape, the arrow would be pointing to the same control point on the dragged object.
When you want to disconnect the arrows from the boxes you just need to drag away the end of the arrow from the box.
Erasing the box results that the attached arrow will remain on the canvas pointing to its original direction.