The best way to understand what you can do with attribute Rules is with an example:
Example #1:Before Status can be changed to "Resolved", the Issue Type must be a valid selection.
To achieve this, under the Status attribute list, locate the Closed - Resolved item and click the icon.
Then select all the valid item selections for the Issue Type attribute.
Now when this case is managed, if a user attempts to "close" the case when the Issue Type is set to "Unassigned" they will be prevented from doing so and will receive a warning indicating that the Issue Type must be selected.
Default Attribute List Item
The list item set as the default when the case is created has a optional rule available - "May not be selected again once changed".
This allows you to specify that once this item has been changed to another value (other than the default), it cannot be set back to the default item. For example, if a case is default to Status=Open, then changed to In-Progress, it will not be possible to change it back to Open.
Determining which items have rules set
You can easily see which list attributes have rules set as an Edit icon will appear beside the item.
Tip: The power behind rules and workflows in general is that you can have multiple workflows in play to suit the exact situation that you require for working a case.
Require a Note
Another common requirement is to require that a note has been added to the case before allowing a case to be closed for example. This can be configured using this setting as illustrated: