The element spanspec is defined in module CALStable.
PurposeFormatting information for a spanned column in a table
A spanspec associates a name with a span between two columns in a table. In the body of the table, cells can refer to the span by name. Cells that refer to a span will span horizontally from the first column to the last column, inclusive.
Cells can also form spans directly, by naming the start and end columns themselves. The added benefit of a spanspec is that it can associate formatting information (such as alignment and table rule specifications) with the span. This information does not need to be repeated then, on each spanning cell.