Use of LB::src_tag and LB::dst_tag

I want to know the use of LB::src_tag and LB::dst_tag.

We have following iRule for SIP persistence with this iRule what exactly does LB::src_tag and LB::dst_tag does.

when SIP_REQUEST {

if { [clientside] } {

LB::src_tag [SIP::call_id]

persist uie [SIP::call_id]

} else {

LB::dst_tag [SIP::call_id]

persist uie [SIP::call_id]

}

}

}

when SIP_RESPONSE {

if { [clientside] } {

LB::src_tag [SIP::call_id]

persist uie [SIP::call_id]

} else {

LB::dst_tag [SIP::call_id]

persist uie [SIP::call_id]

}

}

Thanks

Syed Nazir