See W3C documentation for the XHTML table model.
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 to be aligned horizontally based on the character named in this attribute, for example, alignment on a period for decimal alignment or on an equal sign for alignment of variables.
Horizontal alignment on a particular character requires that three attributes be set in the XML document:
|Text, numbers, or special 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.