Hi Selva,
1.) Via MM we can do all types of stock transfer with stock in transit facility(duration is involved-sending, receiving at different locations), transport charges adding to the receiving plant material cost.
2.) Wherever 2 cities are involved between 2 plants, and in case of excisable stock you have to accompany the stocks with excisable invoice to generate the same we have to have the SD route.
3.) In case of Intercompany stock transfers we have to go via SD route only, because 2 different business units are involved and selling, buying is taken place.
http://help.sap.com/saphelp_erp2004/helpdata/en/4d/2b90dc43ad11d189410000e829fbbd/content.htm
R,
Amala