The bus a sync group's tooltips and crosshairs share hover over, and its brushes the brushed window (spec/10 §4.3, §6.4, D-116, D-117).

A design joins a group with interaction: %{sync: "<group>"} (spec/14 §2.9), and its markup carries data-vis-sync and data-vis-sync-x — the group, and d0,d1,x0,x1,width: the x domain, the chart pixels of its ends and the chart's width, from Visualize.Chart.Frame.sync_attrs/1. VisualizeSync reads them and carries a hover between members as a CustomEvent named vis:sync:<group> on document, whose detail is {x, source}: the pointer's x in domain units (milliseconds since the Unix epoch for a time x), null to clear, and the hook element that published it. Each member maps the value through its own five numbers, so members may differ in width, margin and domain; one whose domain does not hold the value draws nothing.

A brush travels on its own event, vis:sync-brush:<group>, whose detail is {extent, source}: the brushed [lo, hi] in the same domain units, or null to clear. A member draws it clipped to its own domain, and nothing when the two are disjoint.

The object is not exported: TooltipHook, CrosshairHook and BrushHook use it, and a host joins a group through the design key, never through it.