The 'S' series (S2, S3 and S4 camera bags) have the 'TukTop' feature. This allows you to tuck the zips behind either side of the main compartment to keep the bag open and zips out of the way when accessing your gear frequently.‚Äã ‚Äã
Either side of the main compartment, where you tuck the zips, are slim pockets – suitable for a travel guide or camera batteries etc.