When users receive emails and they want to unsubscribe in a single click, we need to authenticate them without login in. We use this random string for that. Look at the "instant unsubscribe" link at the bottom of the emails you receive from B2evolution.net -- you will se the unsubscribe key used there.

Thank you Pascal for this quick reply.
To be honest, I knew what it was for and was to lazy to screen the code to see how it is generated.
What I am doing is generate blog users from another source.
www.mc3lys.fr is the web site of my motorcycle club and I generate users in the blog from the FFM (French Motorcycle Federation) data for our members.
I found out everything I need, except how this key is generated.
So, I understand that I just have to generate a unique random 32 char string.

Regards.

Test it out, I believe if you leave it empty b2evolution will fill it when it needs it.

Will do.
Thanks François (oh and sorry for calling you Pascal ;-(
Do I have to "close" the discussion ?