In context of 8L chassis there are four basic turbos... K03 and K03s on the A3's and two flavours of K04 for the S3... the K03s is a more capable turbo than the K03 but the only realistic difference between K04's is the EGT probe boss for turbo's used on the wideband AMK/BAM engine codes..
VAG used these turbo's across the whole Mk4 chassis range too, SEAT's, Skoda's, VW's etc...
The inline 1.8t 20v (A4's, Passats etc) had different variations of the the K03 turbo and a 'K04' which isn't like the K04 on the transverse engines..
The K03's are transferable across any transverse engine using a K03 but most people try to upgrade to the K03s anyway... You can use a K04 from an AMK/BAM on an APY S3 by plugging the EGT bung but you can't use a K04 from an APY without at least changing the hotside or having a 'bung' welded onto the hotside for the EGT probe..
<tuffty/>