Here you can specify the percentage of periods you want the software to schedule. Leaving the field empty means that the optimisation tool will attempt to schedule all the available periods.
The percentage refers to the periods of the entire school. So if you schedule 10% of a total of 1,000 lessons, Untis selects those 100 lessons that are regarded as to be the most difficult ones. This helps you to find out quickly which lessons are difficult to schedule.