If you rip entire CDs to a single audio file, it is still possible to import metadata. You must have the .cue file produced when the album was ripped or the information must have been saved as a CUESHEET tag in the file. The cue sheet describes the start and stop points of the individual tracks in the file.
The following discussion applies to both the Discogs and MusicBrainz wizards.
The Release Details panel will display an Import to Cue Sheet button if exactly one file is selected in the main window and the file meets the criteria set in Settings - Cue Sheets - Metadata Source.
A good summary of the format and supported tags in cue sheets can be found on Wikipedia. Various settings are available for the function in Settings - Cue Sheets.
The function will attempt to import whatever information can be extracted from the displayed release, preserving the critical information. You can import at most one disc's metadata. A single selected track in the track list describes the desired disc. Note that the cue sheet must have the same number of tracks that are displayed in the selected disc. The existing cue sheet is overwritten.
Note that IRSC codes can potentially be extracted from MusicBrainz. The information is not readily available in the Discogs release data.
When the import is finished, the wizard is not closed. If you wish to import metadata directly to the audio file, select metadata appropriate for the entire album; map the single file to any track; and import the metadata as if you had the individual files.
If an existing cue file is deleted or overwritten an attempt will be made to move the original to the trash.