Hi Bassem,
In addition to the information that Shella and Gavin have provided, each STP user will need to notify the ATO with the new software ID to avoid any error when submitting pay runs. Regarding BMS ID, your client will need to check their MyGov account to check the previous BMS ID. More information regarding your concern can be found here.
Feel free to post again anytime if you require further assistance.
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Best regards,
Doreen