Edit: This is not a duplicate question.
The other question asked how angular momentum remained constant if the distance varied.
This question asks why you can select any point and calculate angular momentum from there, instead of intuitively choosing to calculate the angular momentum with reference to the centre of mass/pivot.
A box is moving with constant velocity in a straight line. (The box is not rotating about its centre of mass)
But apparently, you can set the axis of rotation at any point, and the box will have an angular momentum of r x p (r is perpendicular distance from axis of rotation, p is momentum)
But why can you select the axis of rotation at any point instead of only at a pivot/centre of mass?
Best Answer
Short answer: you can calculate the angular momentum from anywhere you want, as long as the vectors $r$ and $p$ are defined. If what you calculate is useful of easy is another issue, but nothing prevents you from calculating a vector product of two things.