Explanation: Special projects may consider higher standards. For instance, manuscripts or old printed documents would certainly profit from gray-scale scanning or higher resolution. The general rule of thumb is that no significant data should be lost at this stage. A human eye must be able to read the scanned pages as easily as the original. Any further process will rely upon this scan quality. If storage volume and production costs permit, the higher the standards, the better the longevity of the data.
The recommendations for scanning quality are compliant with - or higher than - the scanning standards given by the Digital Library Federation [ ].
Note: a possible choice could be the approach chosen by Gallica [ ]: Always put the text into the minimum ISO A* format into which it fits.