The Music Properties metadata fields apply to music files - the formats that Opus currently supports are MP3 and WMA. Several Music fields also apply to video files as well.
The Increment option in this field lets you assign automatically incrementing track numbers when you have multiple files selected. The track number you enter will be assigned to the first selected file, and the number will be incremented once for each additional file. You can also specify that you want to the track number zero-padded by entering a leading zero in the Track number field.
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Adding Cover Art