'extra' attributes :: What are they for?

[Nap]

Hi

I'm presently installing ISPConfig3.0.4.6 on my server.
During the install, a number of questions are asked. One that is't covered in the manual is:

Please enter the following 'extra' attributes to be sent with your certificate request
A challenge password []:

I presume this is similar to the 'passphrase' that is used in SSH RSA keys?
Where is it used? Who needs to know it? Is it safe to leave blank?

Cheers,
Nap

[Nap]

oops, didn't realise this part of the forum is for Linux, not ISPConfig questions. Sorry.