I was going through my checklist for an upcoming 30k service and I ran across an entry for this in my Haynes. What the heck is it? What does it do and how often should I bother changing it? Says you have to remove left rear wheel to check it

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL