Which pool is used?

I found this example on using LB::select. I want to know how does the system know which pool from which to pick a member?

 

 

when HTTP_REQUEST {

 

log local0. "lbserveraddr: [LB::server addr]"

 

check if serverside connection had previously been established

 

if { [LB::server addr] eq "" }{

 

if no serverside connection had previously been established, force one

 

so the iRule has the info required to insert a destination-specific header

 

eval [LB::select]

 

}

 

switch [LB::server addr] {

 

"172.16.5.80" { HTTP::header replace Host www1.customer.com }

 

"172.16.5.81" { HTTP::header replace Host www2.customer.com }

 

"172.16.5.82" { HTTP::header replace Host www3.customer.com }

 

}

 

}