You can add additional rows to your standard rate if you charge different rates for different groups of guests. You might charge more for larger groups of guests, or offer a better rate for longer stays for instance.
The important thing to remember is that the rates are all for the same time of year. The difference between them is only in the number of guests or length of stay.
If you need to add a special rate for a particular set of dates, then you should add a custom date range.