There is no specific meaning, just a written standard format. What kind of element "丨" is represented in front, for example, {x 丨 x...} represents the X element, and "丨" represents the requirement for the X element.
For example, {x 乸 0≤x≤3, and x is an integer} is actually {0, 1, 2,3}.
Dear Tom:
I'm sorry to hear that you failed in the final math exam, but what worries me most is that you are depressed when you face failure.
As senior t