With a - association, there's the option to choose the 'Navigability'. There are two options: 'object A refers to object B' 'object A object B refer to each other' In Option 1. I can't set association from object B to A. In Option 2, this ís possible. In this view Option 1 is just a limitation of Option 2. Why would I want to choose the former?
The reason to not choose for option 2 are performance related risks. Always use option 1 if you don't need to navigate both ways.
To read more about associations, check this article: https://world.mendix.com/display/refguide4/Associations
And this forum topic: