Type corresponding to UNIT in ABAP program

Hi gurus,

I have a field in table MSEG called meins and its datatype is UNIT and length is 3.Can you please suggest how should be type declaration corresponding to UNIT length 3 in ABAP program.I am facing problem because i have declared a constant of type string length 3 and when i am trying to move data from that constant to IDOC segment feild (MEINS having dataelement as MEINS, i.e. type UNIT 3). It is giving some numeric convertion exception.

Thanks