Fairly certain that nodes get into that category simply by having some kind of routing poller turned on, basically if SW detects the OID's for a routing protocol like BGP or EIGRP then they say its a router. In practice that will probably include lots of l3 switches and potentially other types of devices so it is more a general statement than something you would rely on 100% to know every router in your environment.
You would have to decide how this is done, could be by model #'s, routing protocol existence as suggested, or even just a manually or dynamically set up "Group" of devices. If a group is too formal, you could just set up a custom tag of device type and assign your routers to a type of "router" or something.
So quite a few choices on how you handle it. We do a lot of it with dynamically populated groups, which get automatically assigned based on model # or name/caption. Groups are a bit harder to use in alerts, so sometimes we'll set a custom property based on what group their in to make it easier in alerts.
