Language reference : Attributes : Complex attribute definitions : The keys attribute
   
The keys attribute
A keys attribute consists of one or more space-separated keys. Map authors define keys using a topicref or topicref specialization that contains the “keys” attribute. Each key definition introduces a global identifier for a resource referenced from a map. Keys resolve to the resources given as the href value on the key definition topicref element, to content contained within the key definition topicref element, or both.
The @keys attribute uses the following syntax:
The value of the @keys attribute is one or more space separated key names.
Key names consist of characters that are legal in a URI. The case of key names is significant.
The following characters are prohibited in key names: "{", "}", "[", "]", "/", "#", "?", and whitespace characters.
A key may not resolve to sub-topic elements, although a keyref attribute may do so by combining a key with a sub-topic element id.