The default sequencing within ActiveControl is based on the release date/time of the transports.
For example, if transport A is released at 15:00 on 1st January 2019, and transport B is released at 16:00 on 1st January 2019, and both transports are sitting in an Import Queue, then ActiveControl would ‘order’ Transport A to be imported before transport B.
It is possible to change this default sequencing by hardcoding Dependencies between transports. This is useful if transports were inadvertently released in the wrong order, or indeed when there is a need to ensure sequencing of transports across different SAP systems.
ActiveControl allows for inter-dependencies to be defined between transports in the same system (for example if a functional Customising transport is dependent on a particular Workbench transport), and also for cross-system dependencies between transports being deployed in different SAP development systems (for example, if a BW transport is dependent on a particular ECC transport)
In both cases, these Dependencies are defined on the [Advanced Options] tab of the Transport Form in the SAP GUI, Windows GUI or Web UI.
Technically, when ActiveControl checks a dependency where transport A is dependent on prior import of transport B, it checks whether transport B has been applied to the SAP system that transport A is being applied to, or that transport B has been applied to at least one other SAP system with the same role as the SAP system that transport A is being applied to.