Of every part in the milking cluster, the liner is the one actually touching the animal — it's the flexible rubber sleeve that sits inside the rigid Teat Shell and does the physical work of opening and closing around the teat in rhythm with the pulsation cycle. Because it's flexing thousands of times per session, every session, it's also the component that wears out fastest across the entire system, and a worn liner affects both milking efficiency and udder health directly, since it's in constant physical contact with the animal.
The 27mm sizing refers to the liner's bore diameter, and getting this right for your herd's actual teat size matters — a liner that's too tight or too loose doesn't grip and massage the teat correctly, which can affect both milk-out completeness and, over time, teat condition. Because liners wear with use regardless of how well the rest of the machine is maintained, most dairy operations replace them on a fixed schedule (by milking count or time elapsed) as standard practice, rather than waiting for a visible problem to appear.