This defines a tag as implicit, meaning it can just be an attribute instead of an attribute=value pair. It must be a recognized attribute in the tag definition, or there will be big problems. Use this with caution!
UserTag tagname Implicit attribute value
If you want to set a standard include file to a fixed value by default, but don't want to have to specify [include file="/long/path/to/file"] every time, you can just put:
UserTag include Implicit file file=/long/path/to/file