First I'm not the tech side of my team so excuse my lack of knowledge but I'm passing further the question that I've been asked.
My curiosity is regarding bones and CPU usage, for animations made in Spine then imported into Unity.

Is there a difference in CPU usage, between bones with 0 length and bones with >0 length?
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We're trying to optimize characters and buildings for a 2D game as best as possible without sacrificing too much detail.

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    once There should be no difference in performance depending on the appearance of bones because the bone length is not typically used at runtime except to draw debug lines for the bones.

    If you are interested in the details of each skeleton data format(JSON/Binary), you can find the documentation here:
    http://esotericsoftware.com/spine-json-format
    http://esotericsoftware.com/spine-binary-format

    Basic information on performance is available on the Metrics view page of the Spine User Guide, so I highly recommend reading: http://esotericsoftware.com/spine-metrics

        Misaki Thank you for the answer and documentation

          Misaki the bone length is not typically used at runtime except to draw debug lines for the bones

          Close, but bone length is used for IK and path constraints. Still it makes no difference to performance.