MOSAIC.TXT is a list of images that mosaic will use to construct an image.
These images will be registered and resampled based upon the bigmap you are using. You can use a single image if you'd like to see where the image lands on the bigmap.
JRW Edit: When working with MOSAICX (i.e. the line sample version of MOSAIC) I found the order of MOSAICX.TXT is the first image listed will be "on top". So in the example below, FC21A0014660 could be fully covered by the other images, such that none of the DN values from that image will show up in the mosaic. I assume MOSAIC works the same way.
FC21A0014661.DAT FC21A0016635.DAT FC21A0016636.DAT FC21A0022692.DAT FC21A0023442.DAT FC21A0023449.DAT FC21A0026846.DAT FC21A0026847.DAT FC21A0014660.DAT END