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Hi @alex.van.canneyt.  That is a very complex topic and is very difficult to fully explain without the full knowledge of all those who developed the design software, and how everything is created and works.  Many geometric objects have 'direction' (a direction they are pointing), even when you may not think they would have, and this direction plays a role in assembly constraints.  For instance both a Face object, and a WorkAxis have a direction.  The geometry of a 'planar' Face object is a Plane object, and a Plane object has a property called Normal, which returns a UnitVector, which is used in 3D coordinate space to indicate direction.  The Line geometry of a WorkAxis object also has a property called Direction, which also returns a UnitVector.  This is how you can have constraints that can be Mate vs Flush or Aligned vs Apposed, it all has to do with the natural direction of the selected objects.  And that natural direction is established in how the objects were created.  Similar to how sketched lines all have a start point and an end point, which means there is a start and end, which means direction.  That direction is sometimes important.  If you generated 10 bolds all from the same template bolt, and they were all very similar except for diameter and length, then they would all act the same when replacing one with another, but the replacement bolt does not share the same ancestry, its directions may be different. 

Wesley Crihfield

EESignature

(Not an Autodesk Employee)