Multiple selection of target file(s) provides the ability to override symbology on levels that exist in multiple files. For example, you might have several files that have a level called Border and elements on that level appear in a different color in each file. You can change the color symbology once and the color will be changed in all the selected files.

In the example below, displaying the Level Manager dialog with the relative files selected, Level 1 is blue in BSI300AE9-Shell.dgn and yellow in BSI300S-9-Structural.dgn. In the last figure, both files are selected and the color has been changed to green. The modified Level 1 is marked with a bullet in the Δ column because the color setting is different from the value in the reference source file.