Input service distributor

lets take an example A head office having 4 units A,B,C,D Unit A & B r doing Manufacturing & Unit C& D providing services. Now H.O hires advertising Agency for advertisement of Units A& B products & C& D services. as Ho hires Adv.Agency so agency make a invoice(having Service tax) for Ho & not for units, But Ho is not mfr nor it providing services so The Service tax paid by HO to Agency but how HO can utilize such Service tax so therefore Ho make BILLs/challan for unit ABCD & through this challan Paasses the Service tax credit to ABCD.so here HO is known as Input Service distributor (ISD)