LFBNR Field in MIRO and MIGO

Hi,

This is regarding the (LFBNR) Ref.Doc. field in migo & miro,

We are using 103 and 105 process for migo,

If we display 103 Doc No in MIGO- Ref Doc Field is same , Why??

If we display 105 Doc No In MIGO- Ref Doc field is of Mvt ty 103 Doc no ,

( here 103 mvt type linking with 105 , so no questions.)

If we display 122 Doc no- Ref Doc field is of Mvt ty 103 Doc no,

since returns made against PO, against receipt, 103 Doc type is non valuated ,why its number appears in ref Doc Field in migo

If we process Miro/Credit Note - Ref Doc field is of Mvt ty 103 Doc no,

if we are processing miro then we are processing with valuated GR so 105 doc no should appear correct if i am wrong ??

Please Help me understand about this field , how it behaves??