A container with a transform but no box of its own: a group derives its bounds from its children, and they are positioned relative to it.
<group x={100} y={100}>
{/* children */}
</group>A group takes the common transform (x, y, offsetX, offsetY, rotation, scale, scaleX, scaleY, opacity), composite (blendMode, hidden) and timing (start, end) props.
It has no fill, no width and no height, and takes no paint children: its box is the union of its children's. <effect>, <animation> and <keyframeTrack> children are valid — an effect on a group filters the group as a whole.
A group with no end of its own spans its children: it begins where the earliest one begins and ends where the last one ends, and follows them as they move. Give it an end and it trims them instead — children outside the window do not play. See timing.md.
An empty group, having nothing to span, falls back to the 16-second default.
Groups are what the editor's group action creates and what its ungroup action takes apart, so a group written by hand and one made on the canvas are the same element. A <sequence> is a group with the non-overlap invariant added and its own transform taken away.