If the ATS will receive the service conductors before the house main disconnect, then the ATS must be service rated.
Whether you need to hit a disco with the gen conductors before the ATS depends on where the set mounted CB is. Also see 225.31
If the generator neutral will be solidly grounded, then be sure that your transfer switch also switches the neutral (3 pole) as the bonding of the neutral indicates a separately derived service and requires the extra transfer of the neutral.
Many ATS's have the indication required. The AHJ would have to say it was practical to install the annunciation, most don't.
