A pointer went down: the first starts a drag or a tap, a second finger or pen starts a pinch; a third is ignored.
public void PointerDown(long pointerId, PointerKind kind, double x, double y)
The pointer's identifier (the DOM's pointerId).
What the pointer is.
The x in device-independent pixels from the viewport's left edge.
The y in device-independent pixels from the viewport's top edge.
Spec #33 viewer capability 14.