Merging of cascading attributes

The @cascade attribute can be used to modify the additive nature of attribute cascading, although it does not turn off cascading altogether. The attribute has two predefined values: merge and nomerge.

Indicates that the metadata attributes cascade, and that the values of the metadata attributes are additive. This is the processing default for the @cascade attribute.
Indicates that the metadata attributes cascade, but that they are not additive for <topicref> elements that specify a different value for a specific metadata attribute. If the cascading value for an attribute is already merged based on multiple ancestor elements, that merged value continues to cascade until a new value is encountered. That is, setting cascade="nomerge" does not undo merging that took place on ancestor elements.

If no value is set for the @merge attribute and no value cascades from a containing element, processors SHOULD assume a default of merge.

Implementers MAY define their own custom, implementation-specific tokens for the @merge attribute. To avoid name conflicts between implementations or with future additions to the standard, implementation-specific tokens SHOULD consist of a prefix that gives the name or an abbreviation for the implementation followed by a colon followed by the token or method name. For example, a processor might define the token "appToken:audience" in order to specify cascading and merging behaviors for only the @audience attribute.

The predefined values for the @cascade attribute MUST precede any implementation-specific tokens, for example, cascade="merge appToken:audience".