function cv.glm in library 'boot'

Hi, everyone, 
I have a question regarding function cv.glm in library
'boot'. 
Basically cv.glm can  calculate the estimated K-fold
cross-validation prediction error for generalized
linear models. My question is this: if I am fitting a
logit model, what kind of threshold will it use to
calculate the prediction error (saved in 'delta')? It
will use 0.5 as the threshold or pick a threshold such
that the prediction error reaches its minimum?

Thank you!

Best wishes,
Jun