Sector 21538 was a region of space.

In 2369, a starship mission status listed the activity of Federation starships in sectors 21538 to 23079. This chart was seen in the observation lounge of the USS Enterprise-D. (TNG: "Chain of Command, Part I", display graphic)

This sector was only mentioned in writing.
Sector 21538 was not mentioned on screen, but rather was seen on a viewscreen chart on observation lounge. It was also displayed on a PADD, as shown in the It's A Wrap! sale and auction. [1]
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