iRule to store cookie in table

I found such iRule:

 Limit each client IP address to 20 concurrent connections

when CLIENT_ACCEPTED {
 Check if the subtable has over 20 entries
    if { [table keys -subtable connlimit:[IP::client_addr] -count] >= 20 } {
        reject
    } else {
         Add the client IP:port to the client IP-specific subtable 
         with a max lifetime of 180 seconds
        table set -subtable connlimit:[IP::client_addr] [TCP::client_port] "" 180
    }
}

when CLIENT_CLOSED {
     When the client connection is closed, remove the table entry
    table delete -subtable connlimit:[IP::client_addr] [TCP::client_port]
}

What I don't get is

[table keys -subtable connlimit:[IP::client_addr] -count] >= 20

and

table set -subtable connlimit:[IP::client_addr] [TCP::client_port] "" 180

Counting seems to be performed on subtable named connlimit:[IP::client_addr] but adding keys is performed on subtable that seems to be named connlimit:[IP::client_addr] [TCP::client_port].

What's going on here?

Piotr