Use W3C documentation for the XHTML1.1 table model. (http://www.w3.org/TR/xhtml-modularization/abstract_modules.html#s_tablemodule)
This attribute may be used to place a cell into conceptual categories that can be considered to form axes in an n-dimensional space. User agents may give users access to these categories (e.g., the user may query the user agent for all cells that belong to certain categories, the user agent may present a table in the form of a table of contents, etc.). The value of this attribute is typically a comma-separated list of category names.
Caution: This attribute has not been widely supported.