Attributes commonly used throughout the XHTML Table Model Tag Set, as modified by NCBI to denote 1) whether an alternate form of a table is present and 2) the subject or content type for preservation of semantic integrity
See W3C documentation for the XHTML table model.
Since this is Parameter Entity is defined in the public XHTML Tag Set, it does not follow the naming conventions for this tag set: it is in mixed, not lower case, and the separator character is a period, not a hyphen.
<!ENTITY % Common.attrib "id ID #IMPLIED alternate-form-of IDREF #IMPLIED content-type CDATA #IMPLIED %Core.extra.attrib;" >
XHTMLtablesetup.ent