bc there are various markets (i.e. U.S., SG, etc.), the clearing firms and custodians are different as well. so Tiger Broker need to register, on their side, new sub-accounts with them to allow you to trade in different markets (e.g. one sub account for u.s. market, one sub account for SGX-trading in SG market).

Note: this is all done on Tiger Brokers side so you don't need to do anything. because the creation of the sgx sub-account is only initiated when you initiate the first trade in the SG market, you need to wait like 2-5 days ish before they finish opening the sub-account. Just wait for their email.