This is the approximate percentage of the server tick that has been used already. A value under 100 means there’s time to do more calculations, which can include any pending procs that are still waiting to run on this tick. When the value is over 100, the tick is running long and your world will experience lag.
Keep in mind that sending maps to clients is the last thing that happens during a tick, except for handling any events such as player commands that might arrive before the next tick begins. Therefore in a verb, tick_usage might have a higher value than you would expect to see in a proc that loops and sleeps.