set key iRule question

Hi,

 

I have an RDP pool with an iRule and the following contents (attached to a 3389 VIP):

 

when CLIENT_ACCEPTED {set key [domain servername.company.com 3]}

 

I apologize for the novice question but what does this actually do, the name (opted out) actually resolves to the VIP ip, and the number 3 has me puzzled as well. Any help would be greatly appreciated

 

Thank you

 

-- Jesus