Accounting Document Number

Hi All

am abaper, I have observed that for billing doc. type F2 and accounting doc type RV the same number is generated. where is this done.

1) My requirement is to have a customized number range for accounting doc type. I hope there should be some user exit for this.

Can anyone pl help. I dont have any FI consultant near me. Pl help.

Thanks in advance

Sitharamaraju K