[c-nsp] Cisco 7201 -- Differences between the 12."2" images and the usual 12.4/15 images?

[P C]

I'm deploying a Cisco 7200 series router.  I've used ISRs in the past many
times, but have never touched any 7200 series (or non-ISR for that matter)
hardware.

Can anyone explain to me the purpose of the 12.2 images I see on CCO for
this device?  These images seem to have a different naming and numbering
scheme than that 12.4/15 images.  Normally, if this was any ISR I would just
use 12.4T15(latest) as these seem to be the best combination of stability
and necessary features, but I can't find any clear documentation on what the
12.2s are for.

Thanks!