A derived mosaic dataset is a mosaic dataset that is created from adding other mosaic datasets into it. When populating a derived mosaic dataset, it is okay to use either the Raster Dataset or Table as your raster type. The raster type chosen will load your data in a slightly different way.
If you add your source mosaic datasets as a Raster Dataset raster type, each mosaic dataset is added as one row within your derived mosaic dataset. This means that if there were 3 source mosaic datasets, then only 3 rows will appear within your derived mosaic dataset.
If you add your source mosaic datasets as a Table raster type, then your derived mosaic dataset will add each item from the source, as its own row. This means that if there were a total of 300 mosaic dataset items within the source mosaic datasets, there will be 300 rows within your derived mosaic dataset.