When you apply a master page to a document page, all objects on the master, called master items, appear on the document page. Sometimes you want a specific page to be only slightly different from a master. In this situation you don’t need to re-create the master layout on the page or create a new master. You can override or detach the master item, and other master items on the document page will continue to update with the master.
Note the difference between overriding and detaching master items on a document page:
Attributes you can override for a master page object include strokes, fills, contents of a frame, and any transformations (such as rotating, scaling, shearing or resizing), corner options, text frame options, lock state, transparency, and object effects.
In some instances, you want to override all but a few master items. For example, you may want to override master items such as background images on a document page, but you want to prevent a page-numbering header from being overridden. By preventing the header from being overridden, you can choose the Override All Master Items option to override all master items except for the header.
Master items that do not allow overrides have no frame edge when displayed on the document page. If you prevent a threaded text frame from being overridden, all text frames in that thread have the same setting applied.