What are the applications of bandstop filters?

for...example..you..have..frequency...range...of..f1-f2-f3-f4,

now..your..work..requires..you..to..avoid..frequency..band..of..f2-f3....

what..you..will..do???you..have..to..avoid..or...reject..that..particular..band...so..you..will..

have..to..design..a..filter..circuit..according..to.the..requirement..