Rear derailleurs or mechs come with a wide range of options from the big manufacturers; Mechanical, Electronic, Short Cage, Medium Cage, Long Cage, Clutch, 9/10/11 /12 speed. These options depend on which groupset you have and which cassette ratios you use. Merlin stock derailleurs from Shimano, Campagnolo and Sram for road and MTB. Rear mechs eventually wear out, and springs gets worn from general use over time, this can lead to sloppy / vague shifting.