Why can't I see the 'Include all subject classes' option in Markbook?

The ‘Include all subject classes' option on the Markbook Activity Assessment screen is available to users who are listed as a Subject Department ‘H.O.D.’ or 'Department Administrator’.

‘H.O.D.s’ and ‘Department Administrators’ are defined in TASS.web program Student Admin > Student Records > Setup Information > Student Records Setup > 'Subject Depts.’ tab.