What does "Download GPS assisted data only over Wi-Fi networks" really mean?

Since I checked this option in Settings > Personal > Location, GPS finally works when no data connection is available, eg. when travelling in foreign countries. It's still slower than when a data connection is available, but at least, it does locate me when I'm standing in open skies.

However, I was wondering what this option really means: Does it mean that it will never use A-GPS to download data from the Net even when a data connection is available through GSM *?

IOW, if I'm travelling in my country where a GSM data connection is available, will Android download A-GPS data to speed things up?

Thank you.

• GSM, as opposed to wifi; I don't know a better term