Most recorders allow you to play back another (or the same) title while recording a title. Does the recorder allow this on DVDs without restriction? Some recorders cannot play back a high quality recording on DVD while simultaneously recording a high quality recording to the same DVD, but some recorders can do even this. This feature tells you if the recorder can play back a title on DVD while simultaneously recording to the same DVD in any and all qualities without restrictions.
On some recorders this function is called TimeSlip.