1. What does the small + in the thumbnail for a layer indicate? Is it just indicating a filter was used?
2. What about the small square? Is it just indicating a shape?
Unlike a standard vector object, a Rectangle can be resized on the canvas using the Pointer tool. It can also be given rounded corners, using the Properties panel.
Sometimes, a rectangle will need to be "ungrouped" to perform certain path operations. Selecting the rectangle and applying the Modify > Ungroup command will convert it into a standard path. Within the Layers panel, this will change its thumbnail icon to a standard vector path (Pen tool) icon.