Shapes 

Besides altering bitmap images, Acorn also provides tools to add your own simple vector shapes.  Please note that the only file format that can save your shapes for editing at a later time is the native Acorn file format.

Vector layers (shapes and text) are different from bitmap (or raster) layers.  Vectors revolve around lines and shapes, where as bitmaps are pixel oriented.  For more information on this topic, read here.  You cannot apply filters, drawn on, or erase using the eraser tool to a vector layer unless you have rasterized the layer by selecting Layer ▸ Rasterize Shape Layer.  Once you have rasterized a vector layer you cannot go back and modify the shapes or text without undoing subsequent work.