How do I use the attribute element in the transfer rules?

Attributes are defined in the rule file in order to identify possible values for word characteristics. An example is shown below:
Certain words may have affixes that indicate grammatical number. The attribute above defines the possible values for the number attribute as being sg (singular) or pl (plural). Like lexical categories, these refer to tags in the data stream. Attributes are used within rules to get or set values.