A Group controls one or more Container-s (usually Clip-s, but it can also control other Group-s).
Its children must share the same Timeline, but can otherwise lie in separate Layer-s and have different timings.
To initialise a group, you may want to use group, and similarly use ungroup to dispose of it.
A group will maintain the relative start times of its children, as well as their relative layer priority. Therefore, if one of its children has its start set, all other children will have their start shifted by the same amount. Similarly, if one of its children moves to a new layer, the other children will also change layers to maintain the difference in their layer priorities. For example, if a child moves from a layer with priority 1 to a layer with priority 3, then another child that was in a layer with priority 0 will move to the layer with priority 2.
The start of a group refers to the earliest start time of its children. If the group's start is set, all the children will be shifted equally such that the earliest start time will match the set value. The duration of a group is the difference between the earliest start time and latest end time of its children. If the group's duration is increased, the children whose end time matches the end of the group will be extended accordingly. If it is decreased, then any child whose end time exceeds the new end time will also have their duration decreased accordingly.
A group may span several layers, but for methods such as get_layer_priority and edit a group is considered to have a layer priority that is the highest priority (numerically, the smallest) of all the layers it spans.