Typically you create an import set when in the Discogs or MusicBrainz wizard. There might be times when you wish to manage your sets without running the wizard. You can do so from the Actions menu.
The table displays the metadata which can be imported. The importing of each item type may be enabled or disabled by checking or clearing the Import column. Setting the Import value to - implies that the field should only be imported if the associated value in a file is not empty. ie. current non empty values will not be overwritten. You can also use the table's context menu to set all or selected items to a specific state.
The Manage button displays a panel where you can add, export, load and remove saved import sets. In a typical situation where you are creating a new import set, you would configure the import settings; click Manage; then add a new set.
If you click Done before saving your set, simply use the Actions menu to open the panel again ... your settings are retained until Yate exists.
For more information on the metadata types use the Wizard Release Details links found below.
Discogs & MusicBrainz Import Settings