PropCheck.TargetedPBT.user_nf

You're seeing just the macro user_nf, go back to PropCheck.TargetedPBT module for more information.
Link to this macro

user_nf(generator, nf)

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This uses the neighborhood function nf instead of PropEr's constructed neighborhood function for this generator.

The neighborhood function nf should be of type fun(any(), {Depth :: number(), Temperature::float()} -> any()