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Each element on SDF is defined with respect to a specific reference frame:

  • Links are defined on the parent model's frame
  • Joints are defined on the child link's frame

So let's say you have two links in a model connected by a joint. The links starting configuration is fully described by their own pose elements. The joint doesn't change any of the link poses at startup. What the joint offset changes is how the movement of these links will be constrained once the physics starts acting on them.

Is the length/offset/distance of a newly created joint determined at the moment of it's creation?

Yes, the joint pose is defined the moment the joint is created. The joint does not change how the links are positioned in the world, it just adds a constraint to their movement from then on. I hope this makes sense.