Alpha Convert Node
The Alpha Convert Node converts the alpha channel interpretation of an image.
For compositing and rendering, Premultiplied Alpha is the standard in Blender. Render layers will be premultiplied alpha, and images loaded into rendering or compositing will be converted to this.
If you want to do a compositing operation with straight alpha, the Alpha Convert node can be used. Typically this would be a color correction operation where it might give better results working on RGB channels without alpha. If the alpha is converted to straight in the Compositor, it should be converted back to premultiplied before the Composite Output node, otherwise some artifacts might occur.