Object ID on transform vs shape node

[SonyBoy]

Can anyone explain what the difference is between adding the Object ID attribute to a transform node, and adding it to the shape node? In particular I find this very confusing in how it relates to creating matte elements. You can have a different ID number applied to the transform and the shape of the same object. It seems like the shape node's ID number overrides that of the transform, but I'm not sure. What is the rationale behind having IDs on both? Also, there must be a better way to add this attribute and/or remove it from multiple objects at once? The only means I have found is the drMM script that someone provided, however this doesn't always work, and doesn't remove existing ID Object attributes so can lead to redundancy and confusion in the MM elements it creates (because of the transform vs. shape problem). I'm hoping this problem goes away with cryptomattes (we are waiting to upgrade to 3.6), but in the meantime this continues to be a time consuming headache. Any insight appreciated.

[vlado]

Being able to assign the object ID on a transform allows you to assign it to an entire sub-tree of the scene hierarchy, and then override individual shapes/transforms as needed.

You can also use a V-Ray Object Properies node to assign an Object ID to multiple objects.

Best regards,
Vlado

[SonyBoy]

Thanks Vlado. This sort of makes sense, but doesn't really explain why individual objects can have different (and therefore conflicting) IDs on their transform and shape nodes. Wouldn't it make more sense for it to be possible to add the ID attribute to only the transform OR the shape node? I can't think of any situation where you would want to have both AND to have them be different. I guess everyone's workflow is different, so perhaps there is a use for this, but I only use object IDs when I want unique mattes for every object in the scene.

[vlado]

It would be somewhat complicated to enforce such a restriction on the UI...

Best regards,
Vlado