Mapping Center Answer:

Your ArcMap user interface, including which extensions are enabled, can be saved in a configuration file. By default, they are saved in the Normal.mxt. But you can also save them with each map document. To use the second option, click on Customize on the top bar menu and on the Options tab check the option to Save all customizations to the current document. This will save all the customizations from Normal.mxt to the current document, and all further customizations will also be written only to the document. When opening a document that has this option set, only the customizations in the map document itself are applied; the customizations stored in the Normal.mxt are ignored. Unchecking the Save all customizations to the current document option deletes all the customizations currently stored in the document, and the document will again reference the Normal.mxt. To learn more, read the help topic called Fundamentals of saving your customizations.