A Constant attribute can never change. It's practically the same thing as assigning Text to a block, except you can export this text to an external file. Let's set our Furniture Type attribute to be a Constant attribute that says Sofa. This object will always be a sofa. Later you might choose to extract the furniture types into an external file.
Verified attributes are a rare breed as well.
If you had a complicated model number or a key piece of information that absolutely must be accurate, you might choose to use the Verify mode when creating your attributes. When inserting a block with a verified attribute, you'll be prompted twice to verify that the information you provided was correct. Just for the heck of it, let's set our Manufacturer attribute to be verified. If you have an attribute that defaults to a specific value 90 percent of the time, you might consider making it a Preset attribute. This mode is similar to Constant in that you won't be prompted for the attribute value, but it's changeable after the fact.