Section 508 Web Standards
(h) Markup shall be used to associate data cells and header cells for data tables that have two or more logical levels of row or column headers.
What does it mean?
When the data tables are complex and have more than one level of row or column headers, it is necessary to add special attributes to these headers so they correspond to data cells.
How to evaluate
All data cells are associated with the appropriate headers and attributes (ID). The screen reader appropriately identifies row and column headers and is able to read the data in logical order, from left to right across each row. The table does not contain spanned rows or columns. Other attributes are used such as Caption to display a title above the table and Summary to explain the results of the table. It may be necessary to create several data tables instead of one large one if the large one becomes too complex for screen readers to handle.
For more information
Design Resources