audio - centered information is stored on a unitary medium by using a table - of - contents mechanism that specifies an actual configuration of various audio items on the medium . in particular , each one of a set of one or more track areas gets at least two mutually logically conforming sub - tocs assigned . this allows to retrieve any constituent sub - toc part from at least any correct copy of the sub - tocs . furthermore , one or more master - tocs are provided for specifically pointing to each of the sub - tocs .