The transmitter itself has a dynamic impedance in the 10s of ohms range; this is intended to drive the line to nearly the supply voltage, ensuring maximum signal at the far end. The far end then gets terminated, and somewhat less than the supply voltage is available there (the loss being due only to line losses).
With source termination, the impedance of the line is the only load; this is good for saving power over short distances, but the risetime falls over more rapidly with distance, the line isn't suitable for multidrop connection (intermediate receivers will get a stepped edge), and the line also won't return to zero (which makes RS-485 problematic -- hence this is more of an RS-422 option, i.e., always-on transmitters).
Tim