The sort order determines the order the entries are printed on the report.

Date Select Date if you want to sort the entries by the date on which the email statement was sent, deleted, or failed to send. The entries for each activity date will print in order based on the time at which the activity was performed.
Primary Timekeeper Select Primary Timekeeper if you want to sort the entries by the Primary Timekeeper selected for each client. Each timekeeper's entries will print in order based on the date and time at which the activity was performed.
Client ID Select Client ID if you want to sort the entries by the Client ID. Each client's entries will print in order based on the date and time at which the activity was performed.