In the diagram, Z1 and Z2 are the impedence of some combinations of L, R and C. The voltage input is placed across AB, and the output voltage is measured across CD. Z1 and Z2 can be designed to make the circuit a filter for output voltage V_CD. However, if we really place a load across CD, it will have an impedence Z_CD, the whole circuit will be affected and the output behavior V_CD will change. Then, the circuit cannot act as a filter anymore if we place any load (at least not as the same filter when Z_CD is not there). How can we say in the first place that the circuit act as a filter? The "filter" only "works" in the absent of load. How useful is LRC circuit as a filter then?