While creating a custom session, the amount of groups can be specified as well as a Pregenerated Grouping.
If we stay with the example of the trust game mentioned in the Group Structure Tr.1 and Te.1 will be grouped together by default throughout the whole experiment and this is what the default pregenerated grouping does.
The random grouping on the other hand generates a new, random grouping for each repetition of each stepgroup. Tr.1 might be grouped with Te.2 in round 1 of the experiment and with Te.6 in round 2.
When an individual experiment without group interaction is conducted, no grouping is required and none should be chosen for the pregenerated grouping, as this saves some loading time, which is required for group creation.