Depending on the context, the DITA table layout accepts CALS tables, simple tables, and choice tables.
In the CALS table model, you can specify column widths using the colwidth attribute of the associated colspec element. The values can be fixed or proportional. By default, when you insert, drag and drop, or copy/paste a column, the value of the colwidth attribute is 1*.
Also the colsep and rowsep attributes are supported. These control the way separators are painted between the table cells.
Figure: CALS table in DITA
The simple tables accept only relative column width specifications by using the relcolwidth attribute of the simpletable element.
Figure: DITA simple table
You can insert choice tables in DITA tasks either using the Content Completion Assistant or using the toolbar and contextual menu actions.