A number of the DITA table types are specializations of the simpletable element, if you create additional specializations, they need to be identified to open properly in DITA-FMx.
When simpletable-based elements are encountered during the import process, FrameMaker needs to be able to count the number of columns in each table. This information is typically stored in attributes within the table element, but the DITA specification does not provide this type of attribute for simpletable-based tables.
The DITA-FMx Options dialog provides access to the Element Mapping dialog which defines the structure of the default simpletable-based tables, and allows you to define the structure of any specializations you create.