Mark the start of each session on the disc.
The process of writing a lead-in and lead-out is called disc fixation and it generates a new disc session.
On CD media it is the area that contains the TOC data and precedes each program area. The main channel in the Lead-in area contains audio or data null information. This area is coded as track zero. The Q Sub-channel in this area is coded with the Table of Contents information.
The DVD Lead-in area is the area comprising physical sectors 1.2 mm wide or more adjacent to the inside of the Data area. The area contains the Control data and precedes the Data area.