Can someone tell me teh meaning for the Irule and how to write similar ones?

The irule parses the CLIENTID from the URI parameters. Then it looks up the uri in a datagroup called itemCodes_int4.

If URI is found in datagroup, grab value associated with that URI, (redirection location).

If URI is found and its associated redirection location is also not empty, and the CLIENTID was found redirect user to found redirection location with CLIENTID attached.

If URI is found and its associated redirection location is also not empty, but CLIENTID not found, send user to new redirection location without a CLIENTID.

In both cases it logs who was redirected and where to in the /var/log/ltm file.

Here is the irule properly posted:

when HTTP_REQUEST { 
 set CID [findstr [string tolower [HTTP::uri]] "clientid=" 9]
 if { [class match [string tolower [HTTP::uri]] contains itemCodes_int4] } { 
    set redirect_location [class match -value [string tolower [HTTP::uri]] contains itemCodes_int4]
    if {$redirect_location ne "" and $CID ne ""} { 
        HTTP::redirect "https://$redirect_location/app/home?clientId=$CID" 
        log local0. "1. [IP::client_addr] requested [HTTP::uri] sent to https://$redirect_location/app/home?clientId=$CID" 
    } elseif { $redirect_location ne "" and $CID eq "" } { 
        HTTP::redirect "https://$redirect_location" 
        log local0. "2. [IP::client_addr] requested [HTTP::uri] sent to https://$redirect_location"
    } 
    unset redirect_location CID
 }
}

HTH