The Plant Booster device uses a control spring to counteract the tendency to close under load.  While the device physically will not fail until a tension load of about 70lbs has been applied, it will start to deform excessively at about 50lbs, and beyond that load the control spring will not be able to counter the load.  The device will therefor simply shut the flow at about 55 - 60lbs, and eventually break at the hook at about 70lbs.
Therefor, when a Plant Booster device used in the conventional way for controlling water to a hanging basket the maximum recommended weight at which the device will still maintain proper control of water flow is 40lbs.   The specific Nylon formula used for the device is stable in strength up to about 200deg C, and therefor normal outdoor temperatures should not affect this maximum weight.  There is obviously some margin in these numbers.
Using the simple principle that in a hanging basket with three strands, the weight the basket is equally carried by each strand.  Therefor is the Plant Booster device limit is 40lbs, and if the Plant Booster device is installed in one of the three strands as pictured below, then the maximum weight of a hanging basket would be 120lbs.