You can activate the substitution time grid in the substitution mode via "Settings | Time Grid".
Here you can define the periods in which substitution shall take place at your school. In some schools it is common that the last periods in the morning or periods in the afternoon are not substituted at all but cancelled.
Mark all periods in which substitutions shall take place and click on the button <Substitution>.
In our example, substitutions take place from the first to the fifth period. Periods after the fifth period are automatically cancelled. If you wish to plan a substitution e.g. in the sixth period, you can change the cancellation to a "normal" substitution by assigning a substitute teacher. On the other hand, you can cancel e.g. a first period although it is marked as to be substituted in the substitution time grid.

Hint: Class time grid
Apart from this global substitution time grid which is valid for the whole school, you can adjust the substitution time grid of classes individually. To do this open "Master Data | Classes", click on the button <Class time grid>
and activate the tab "Substitute".