What is the " 'aux' pad" ?

In GIMP Python programing, I tried to apply gegl:multiply filter to a layer non-destructively, but it failed and following message was displayed.

“drawable_append_filter_private_invoker: effects with an ‘aux’ pad cannot be applied non-destructively.”

I’ve tried looking into " ‘aux’ pad" but I don’t quite understand it. Is there anyone who can explain this concept plainly to me?

In Gegl filters an aux pad is a parameter layer/drawable (like for instance a bump or a displace map).

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Thank you.