DESelect allows you to select which fields from your Selected Data Extensions will populate the fields of your Target Data Extension by creating mappings.
DESelect is responsible for translating your selections to SQL. In SQL not all field type conversions are allowed. To prevent you from using invalid conversions, DESelect does not allow all mappings depending on the field types you are trying to map.
In the table below you can see all the filed type mappings that DESelect does not consider compatible:
|Selected Data Extension / Custom Value field
|Target Data Extension field