By default xSQL Data Compare marks all the table pairs that were found to be comparable for inclusion in the data comparison. You can choose to include/exclude certain table pairs by checking or un-checking the "Sync" checkbox next to each table pair.
When you click on a table pair on the left panel the "Selected Fields" tab on the right panel will display the list of mapped columns for the given pair of tables. The columns are mapped based on the name and the type. The columns must have same name and type to be comparable otherwise they will not be mapped together and will not display on the "Selected Fields" tab at all. By default all the mapped columns have been marked for inclusion but you can choose to include or exclude any of the fields by checking or unchecking the "Include" checkbox next to each column pair.
Important: note that the column(s) that are part of the key that has been selected to be used as the comparison key will always be included in the comparison regardless of whether you have checked or un-checked them in the "Selected Fields" tab.