TPS541620 Buck converter IC - Feedback loop
This technique is usually found on regulator evaluation boards. It allows to break the feedback loop and do stability testing by injecting signals while still keeping the feedback divider and forward compensation capacitor in the path of injected signal. That is why the name is Rinj, and it is 50 ohms as typically lab equipment and coaxial cables use 50 ohms impedance. 50 ohms is very little compared to the impedance of the feeback divider so it has almost no effect during normal operation.