The lead in is an area reserved at the beginning of CDR discs. The lead in is blank until the TOC (table of contents) is written. It occupies 6750 blocks, which are also known as CD frames, in the first .4 millimeters of the disc. (Incidentally the “first” part of the disc is on the inside – discs are written and read from the inside out.) Lead outs are written between each session. If you burn your disc as a Red Book compliant disc-at-once session there is a lead out at the end of that session. In a multisession, or track-at-once disc, there are lead outs between each burn followed by a “pre-gap,” which prepares the disc for the subsequent session.