Table Of Contents.
Every CD has a TOC and it describes the CD layout (Tracks and Sessions)

DVDs don't have a TOC.  The layout is recorded in a different way (DVD Structures).
However, on a ReadTOC command the DVD unit will still respond with valid data.  The DVD unit will translate the DVD structure data to a CD TOC before reporting to the host.

Both TOC and DVD Structure are located in the Lead-in of a session.