Here’s the relevant part of the docs:
The probability of connected to a vertex depends on the degree k of that vertex, on \alpha (which is the same as power) and on a (which is the same as zero.appeal).
k is the in-degree in the directed case, and the total degree in the undirected case. If out.pref is set to TRUE then k is the total degree even in the undirected case.
I hope this answers your question.