About BufferExtmarkByID function's return value

[yyy33]

What is the meaning of pos in the return value of BufferExtmarkByID function? When I pass the details(true) parameter, the returned length is 3, and when I pass the details(false) parameter, the returned length is 2. I know the first two elements are the start coordinates, but what does the third element mean. Why I don't get the end coordinate when I pass the details(true) parameter, but I can get the end coordinate in vimscript

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