ref.df in mgcv gam

Hello Group.

Regarding degrees of freedom in GAM models fit by mgcv, I do understand the
estimated df, but what does "ref.df" in the summary output mean?

I didn't see an explanation in Wood (2017) or find it in the function
code.  The answer may be on page 222 of

Wood SN. On p-values for smooth components of an extended generalized
additive model. Biometrika 2013;100(1):221-228

or thereafter, but I am not a statistician.  I could use help understanding
what the reference degrees of freedom are.

Thank you,

Susan

Susan Elias
Vector-borne Disease Lab, Maine Medical Center Research Institute

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