Whether you want to make things easier for your users or need them to stay within boundaries, you can find use of the predefined values.
The User page is an interface for users to enter their information. You can access its' settings on the User info site.
If you want to get into Predefined values settings, first switch it On and click on Define.
You'll have an option to add the actual predefined values and their order on this screen.
After you save your progress, this is what your users will see when they will try to enter data into the form - a range of predefined options with a possibility to enter their own value.
You also have an option to set the values at On (strict). This option will let users choose only exclusively from your predefined values.