With the block substitution you can assign one substitute teacher to several open substitutions similarly to the substitution coupling. If one teacher is absent for a longer period of time then you can assign one specific teacher always for the same periods over several weeks with the block substitution.
to assign one teacher to several substitutions you have to mark the respective substitution lines with Ctrl+Click.

Now you can enter the short name of the substitute teacher either via typing the short name or via the substitution suggestion.