Most applications do not need to call GetFieldData. TField objects call this method to implement their GetData method. 

The Field or FieldNo parameter indicates the field whose data should be fetched. Field specifies the component itself, while FieldNo indicates its field number. The Buffer parameter is a memory buffer with sufficient space to accept the value of the field as it exists in the database (unformatted and untranslated). NativeFormat indicates whether the dataset fetches the field in the library's native format for the field type. When NativeFormat is false, the dataset must convert the field value to the native type. This allows the field to handle data from different types of datasets (ADO-based, BDE-based, and so on) in a uniform manner. 

GetFieldData returns a value that indicates whether the data was successfully fetched. 

GetFieldData returns true if the buffer is successfully filled with the field's data, and false if the data could not be fetched. 

GetBlobFieldData 

FieldNo