There are three options you can choose from on the right side under'Selection':
New substitutions only
if you check this option then only the substitutions which have been created since you opened Untis last are shown.
Don't print absent teachers
With this option you can avoid that absent teachers and classes are printed with the substitution list. This saves a lot of precious space.
Also elements without substitutions
Here elements are listed even though they are not affected by any substitutions. To this end the format needs to be structured in headings (teachers or classes).