This option on the Detail View page lets you see how much has already been recorded on rewritable media (such as your CD-RW or MiniDisc music mixes).  Now you can know which still have enough space left!  This option will calculate the timing and number of tracks for each of the media labels and directories, and show them sorted in order of most space left.

Any entries shown in red indicate that at least one album on the target media had no track timing, so 3 minutes per track (plus inter-track gap of 2 seconds) has been used as an estimate, which of course could be way off so such target media's actual remaining space should be checked before recording.