Specifies the TCB of which this classifier element is a part. Lower numbers indicate an element that belongs to a classifier that is part of a TCB that is, at least conceptually, applied to traffic before those with higher numbers - this is necessary to resolve ambiguity in cases where different TCBs contain filters that overlap with each other.

A manager wanting to create a new TCB should either first search this table for existing entries and pick a value for this variable that is not currently represented - some form of pseudo- random choice is likely to minimise collisions. After successful creation of a conceptual row using the chosen value, the manager should check again that there are no other rows with this value that have been created by a different manager that could, potentially, interfere with the classifier elements that are desired.