What's the difference between a grouped train and a fixed grouped train?

The difference between a flexible train and a fixed train is that,

Flexible marshalling transportation organization refers to the transportation organization technology that flexibly changes the length of train marshalling at the railway station in the depot to achieve the best coordination between passenger demand and capacity according to the characteristics of passenger flow in different sections and time periods of urban rail transit and under the condition of ensuring high train service frequency. It is one of the important transportation organization ways to solve the imbalance of time distribution. Therefore, the flexible marshalling transportation organization technology can not only better match the passenger flow with the energy and force of the train, but also avoid the no-load traction power consumption caused by the frequent entry and exit of the train depot based on station de-marshalling, and reduce the traction power consumption when the train is fully loaded in the fixed marshalling mode, thus improving the transportation economic advantage. Flexible grouping of EMU trains.