IDX files can have a variety of uses, here are some of them:
A file with the IDX extension can be a movie and video subtitle file, and is used to contain the text to be displayed in the subtitles.
They are similar to other subtitle formats like SRT and SUB, and are sometimes called VobSub files. You can open subtitles IDX files with VLC Player.
IDX files are also used for Navigation POI files.
In this case, the GPS devices store POIs in the IDX file and the device can refer to it during a trip.
In another use, IDX files are just general index files, that a program creates to refer to faster functions, such as searching through large numbers of files.
Another similar index-related file format that uses the IDX file extension is Outlook Express Mailbox Index.
In this case, the program stores an index of messages taken from an MBX (Outlook Express Mailbox) file.
The IDX file is required to import old mailboxes into Outlook Express 5 and later.
For more general information about files, file extensions and registry you can read one of the following articles:
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