The direction is not a meaningful attribute for core links. It is simply a handful way to make links easily readable and therefore they are brings bringing some arbitrarity when a new link type is created. However, the direction is a critical concept in Links hierarchy since it only rewarded linked issues that contribute to their ancestors in the hierarchy and finally the root issue.
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This new feature allows you to decide in which direction the links contribute to the progress roll-up.
Let's see an example:
from a board, you can split an issue...
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