When the week's spend reaches 80% of the maximum weekly budget, you get a notification (at most one every 12 hours, so it doesn't bother you). The option Allow exceeding the limit if performance is excellent, in the Budget step, changes how the ceiling works. If the account's current ROAS is above the top 10% of its own history, the system can spend up to 20% above the weekly limit to take advantage of the good moment. It is off by default. Without it, the limit is strictly enforced, and when it is exceeded, all managed campaigns go to the minimum budget until the week resets. Turn it on only if you are comfortable going over the ceiling when the results clearly justify it.