$.ui.mouse._mouseDistanceMet in ui.core.js

$.ui.mouse._mouseDistanceMet in ui.core.js


Hi,
I study jquery-ui-1.7.1.custom/development-bundle/demos/draggable/
default.html.
What is the purpose of $.ui.mouse._mouseDistanceMet in ui.core.js? And
why is $.ui.mouse.defaults.distance 1 (and not 0)?
Suppose I do a mousedown on the div#draggable, followed by a
mousemove. When should $.ui.draggable.prototype._mouseStart be called?
Thanks.