This is the OEM Donut Gasket which fits between the downpipe and midpipe on downpipes using the stock 2.5″ donut flange.  This includes the COBB Downpipe so if you have purchased that downpipe you will need this for installation.  This part can come in handy if your gasket gets lost or damaged during installation or you simply want to install a fresh one just in case. While most aftermarket exhaust systems use a flat style 3″ gasket, if you plan to maintain the stock cat-back with one of these you will need an adapter which requires the use of this gasket.