What does these F5 iRules mean?

I have no knowledge of F5 and trying to read these rules. If some one can help me understand them.

Following rules is reading HTTP request and (defining variable INTRSSN?) getting a node and saving it in a persistence table.

when HTTP_REQUEST {
if { ( [HTTP::method] eq "POST") and
     ( [HTTP::path] equals "/webserv/Interaction") and
     ( [HTTP::header value "Content-Length"] < 1024 ) }{
     #Debugging Purpose 
     #log local0. "First Request: [HTTP::uri]"
    HTTP::collect [HTTP::header Content-Length] 
    if { [info exists "INTRSSN"] }{
    set IntrExist [persist lookup uie $INTRSSN node]
    #log local0. "Response check if exist $IntrExist"
    if {($IntrExist != "")}{
    node $IntrExist
    }
    }
}
}

This rule will read the HTTP request and extract a specific tag value is put it in INTRSSN variable. This variable will be saved in persistence table.

when HTTP_REQUEST_DATA {
        if { ( [HTTP::path] equals "/webserv/Interaction") and
         ( [HTTP::header value "Content-Length"] < 1024 ) }{
        set INTRSSN [findstr [HTTP::payload] "<soap1:sessionID>" 17 "<"]}
        if { $INTRSSN != "" } {
            #Debugging Purpose 
            #log local0. "SOAP Session ID: $INTRSSN"
            catch { persist uie "$INTRSSN"}
            #log local0. "Request_Data $INTRSSN"
        }
} 

I did not understand this event.

when HTTP_RESPONSE {
    if { [HTTP::header "Content-Type" ] equals "text/xml" }{
    set resdata [HTTP::payload]
    set INTRSSN [findstr $resdata "<sessionID>" 11 "<"] 
    if { $INTRSSN != "" } {      
        #Debugging Purpose 
        #log local0. "Found sessionID on Response: $INTRSSN in SOAP response from: [LB::server addr]"
        #log local0. "Interaction $INTRSSN"
        catch {persist add uie $INTRSSN 600}
    }
}
}

The HTTP_RESPONSE portion attempts to read the XML response and also extract a specific tag value, put it in the $INTRSSN value and save/update a persistence record.

Basically, the whole iRule put together is a way of mapping a specific field within the HTTP body to use for persistence (ensuring the connection goes to the same backend server for the life of the connection).