Service item id with 50001 series replicated from SRM to ECC6 PO

We are implementing SRM7 EHP1 with ECC6 EHP5 backend on a De-couple scenario.

When we create shopping cart with services items and subsequently process to backend PO, I notice the PO which replicated to backend ECC having line "item no." at overview for the services is assigned with 50001 item no. instead of the usual 1, 2, 3, 4, 5 item no.

Additionally, I also notice if I create PR in ECC6 and transfer to SRM as External Requirement, the item no. assigned in EXT_DEM_POSID carry 5000100010 which looks like a concatenate prefix "50001" and service spec item no. "00010"

Where does the prefix 50001 comes from?

Cheers,

Tom