See documentation for the HTML 4.0 (XHTML) table model or OASIS (CALS) Exchange table model (XML version), dependent upon element involved.
Specification of the character to be used in horizontal alignment of the contents of an entry in a table. The content of a table data or table header element is horizontally aligned based on the character named in this attribute, for example, a period for decimal alignment or an equal sign for alignment of variables
Alignment on a particular character requires that three attributes be set in the document:
|Data characters||When used in conjunction with align="char", the contents of the table entry are aligned relative to the character specified by this attribute.|
|Restriction: This attribute may be specified if the element is used.|