The values in the dropdown lists throughout the system are controlled by your administrator, but (with the appropriate permission) you can add values on-the-fly when you are entering data if the default list does not include an appropriate value.
To add a value to a list, click the + to the left of the field.
In the Dropdown Name field enter the value that you want to add to the dropdown list, and optionally enter a Description, e.g. the reason for adding the value.
The administrator can view and edit this and other dropdown values in the Administrator module.