Pipe (or Vertical Bar) operator

What does the pipe character do in a search.

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In 1990s, Finland had the highest incidence of diabetes and cow’s milk consumption in the world. Researchers compared levels of incompletely digested cow’s milk protein (Bovine Serum Albumin – BSA) in 142 diabetic children.
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With the Boolean Mode on, if I search on cow | milk, on the above text, then cow and milk are highlighted. What does the pipe operator do?

Cow, cow’s and milk appear many, many times in my posts but there are only three pages that return results for cow | milk.

Thank you.
Richard