The CUSTOMIZATION tag displays the value from the Customization table for the customization key specified. This may seem very similar to the tags that display values from fields, but the structure of the Customization table is slightly different, so this syntax helps keep things consistent. Common uses for this tag would be if you wanted to display any of the WebDefaultX values as text to your users or explain UserRequestLimits with tags instead of the actual numbers so that you could change them in the future.
An example of how this tag would display in the HTML:
Remember that this tag displays the value from Customization and not the specific value for that user or transaction, which may have been overridden with a new value.