Painting with Gradients or Textures on Bitmap Layers
When drawing on a vector layer, each stroke is its own vector drawing object, and can be manipulated independently at any time unless you flatten or group your strokes together. When drawing on a bitmap layer, most modifications are immediately merged into the artwork, making it a single bitmap drawing that can only be drawn on, painted on or erased. One exception to this rule is when you paint a zone with a gradient or texture.
When using the Paint tool to fill a zone in a bitmap layer with a gradient or texture, a temporary vector object in the shape of the fill is created, and contains the fill. This allows you to use the Edit Gradient/Texture tool on this vector shape to tweak your gradient or texture so that it has the intended position, angle and scale. Then, the next modification you make to your bitmap layer—be it with a drawing tool, or by pasting artwork—will cause the gradient/texture to merge with your artwork, making it no longer possible to tweak it with the Edit Gradient/Texture tool from that point on. Hence, it is important to make sure your gradients or textures look exactly as you intend them to before continuing to work on your drawing.
NOTE Certain tools, such as the Text or Cutter tool, will not cause your gradient/texture to merge with your artwork.