TID inbound to SAP

Hi ,

When we send an IDOC to SAP system we send a TID along with to gurantee that data is send only once. SAP also generate its own TID.

But if i manually send a TID (eg.123) from my external system then also SAP generates its own TID as per my understanding.

My question is where this TID that is passed from the external system is stored in SAP or where the mapping is done between the TID send from external system and SAP TID?

Thanks

Akriti