That would be CTMS holding (Central Traffic Management System), which is a big computer program run by Airservices that allocates landing times and works back from that to work out ground holds for aircraft that have not yet taken off that are inbound to a slot-constrained port (ie SYD). The theory is that delays caused by a reduced landing acceptance rate can be managed on the ground rather than by using airborne holding.
The reduced landing rate might be for weather (single-runway ops for example), or for some other restriction (staffing?).
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