Why are some days greyed out when selecting an end date in a leave application?

This applies to all Deel Local Payroll powered by payspace payroll editions
Some days will be greyed out when selecting an end date in a leave application, depending on the number of days inserted in the the below advance option on the specified leave type (in this example, Standard Leave - Annual Leave)
Navigate > Leave > Leave Applications
Navigate > Config > Human Resources > Leave Administration > Leave Scheme Parameters > advanced options