What is the difference between pan-sharpened and bundle imagery?

Pan-sharpening is a process that "sharpens" up an optical image after it has been collected

Pan-sharpened imagery uses the panchromatic (a single visible) channel at very high resolution (for example 50cm pixel size) to enhance the resolution of individual bands (or multispectral channels, including red, green and blue) that are sampled at less high resolution (for example 2.0m). It is "sharpening" the colour image, if you like. So you would get all images at 50cm in the case of our example (which might be red, green, blue and near infrared). Red, green and blue can then be combined to get a 50cm colour image of the location of interest. Pan-sharpened imagery is useful when the application requires the best visual product for interpretation, as well as for a number of algorithmic applications in which the best spatial resolution is required.
Bundle imagery is simply the separate bands made available as a batch at their original resolutions (e.g. panchromatic at 50cm along with red, green, blue and near infrared at 2.0m). This is useful if you want the "unadulterated" multi-spectral imagery for analysis.
The imagery below demonstrates the difference between a Bundle (Fig A) and a Pan-sharpened product (Fig B).
Fig A - Bundle: True colour @ 2 metres resolution.
Fig B - Pan-sharpened @ 0.5 metres

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