The C8BISRVR_APP flag is to indicate you want to install components from the Cognos Business Intelligence Service set. It's not readily apparent on a basic BI Server installation but there are a number of optional components such as Cognos Transformer and Cognos PowerPlay which can also be installed, which have separate sections.
The reason for this separation becomes obvious when you look at the response.ats file which is included in any Fix Pack installer, as it lists numerous entries in the file for different Cognos products. Each one of those entries has a corresponding _APP entry.