The "christmas tree" is hooked into the LMI (Load Moment Indicator). The crane has a pressure transducer on the boom cylinder that shows/ calculates weight, a boom length from the reel, and boom angle from a angle sensor. It calculates out the "load moment" - i.e. weight on the hook vs capacity. It displays it as a scaling green lights as the load increases, yellow is in the "warning" area- typically after 85-90% of capacity. Red is limit- no go area.
At the red it will lock out the following functions- boom down, telescope out, and winch up. It will allow you to telescope in, winch down, and boom up which will all should gain you more chart or get rid of weight. The boom up being allowed is the dangerous one - it can get a guy in trouble because you may actually be gaining weight when standing a tank or such in boom up.