All images and clips have a premultiplication state. This is used to indicate how the image should interpret RGB (or YUV) pixels, with respect to alpha. The premultiplication state can be...
kOfxImageOpaque- the image is opaque and so has no premultiplication state, but the alpha component in all pixels is set to the white point,
kOfxImagePreMultiplied- the image is premultiplied by it's alpha,
kOfxImageUnPreMultiplied- the image is unpremultiplied.
This document won't go into the details of premultiplication, but will simply state that OFX takes notice of it and flags images and clips accordingly.
The premultiplication state of a clip is constant over the entire duration of that clip.