Filtering only shows rows that match a particular rule, any active filters are shown in the status bar. If you run an autocorrection task it will only be performed on the visible songs. There are currently filters available for the MB Recording Id and Acoustid, only one of each filter type can be enabled at the same time.
When enabled only songs containing a Recording Id field that is the same as at least one other song are displayed. This function is very useful for finding and possibly deleting duplicate versions of the same song. But please note this field was previously described in Jaikoz as the MB Unique Id because the id uniquely identified a track on a particular release, however since MusicBrainz NGS this id now only identifies a unique recording, the same exact recording can be found on multiple different albums, each song would have the same Recording Id but a different Release Id. In fact to accurately identify a particular track on a particular release, the Recording Id, Release Id, Disc no and track no are required.
When enabled only songs containing a Recording Id field are shown. This is useful for listing songs that have been matched to MusicBrainz.
When enabled only songs that do not contain a MusicBrainz Recording Id field are shown. This function is useful for finding tracks that have not been matched to MusicBrainz, that may need matching manually
When enabled only songs containing a Acoustid fingerprint that is the same as at least one other song are displayed, filter status is also shown in the status bar. This function is very useful for finding and possibly deleting duplicate versions of the same song.
When enabled only songs containing an Acoustid are shown. This is useful for listing tracks that have had an Acoustid fingerprint created.
When enabled only songs that do not contain a Acoustid field. This is useful for finding tracks that have not had an Acoustid fingerprint created.