This is a required field.
If you select Time for the activity type, you must then specify a rate for the activity: No activity rate, a Staff rate, or a Fixed rate.
If you select No activity rate, there will be no default rate for the activity, and you must enter the rate manually during time & expense entry.
If you select Expense for the activity type, you can then specify a Unit Price and Unit Cost for the activity. The activity will be available on the Expense tab of the Time & Expense Entry screen.