If you select Copy column for Method, the Source Data list contains only the sending table columns whose datatypes are compatible with the Target Column value. If the datatype mappings are compatible but possibly problematic or may require data truncation or rounding, the Connect CDC Director warns you (when you click OK or at model validation time) but lets you continue.
To determine whether datatypes are compatible, the Connect CDC Director uses a matrix of generic categories instead of comparing every pair of DBMS SQL datatypes. For more information about Connect CDC datatype categories and conversion rules, see the System Reference.