procedure setField (fieldno : Integer; s : string; value : double);

Function setField() puts values to a field in memory before writing a record to a disc.
There are different versions of setField: you can pass a double value, a string or both. In last case setField will use a value which is more suitable for this type of field.

begin
    // your code
    dbf.clearRecord();
    dbf.setFieldString(0, 'string value');
    dbf.setFieldDouble(1, 123.45);
    dbf.setField(2, 'the string', 666.66);
    dbf.writeRecord(0);
    // your code
end;