Because classful routing protocols do not advertise subnet mask information, the routing updates simply list subnet numbers but no accompanying mask.  A router receiving a routing update with a classful routing protocol looks at the subnet number in the update, but the router must make some assumptions about what mask is associated with the subnet.  Classful routing protocols require a static-length subnet mask (SLSM) throughout each classful network so that each router can then reasonably assume that the mask configured for its own interfaces is the same mask used throughout that classful network.  When a router has interfaces in more than one Class A, B, or C network, it can advertise a single route for an entire Class A, B, or C network into the other classful network.  This feature is called autosummarization and is described as follows:

When advertised on an interface whose IP address is not in network X, routes related to subnets in network X are summarized and advertised as one route.  That route is for the entire Class A, B, or C network X.

Classful routing protocols expect autosummarization to occur.