Provisioned capacity is either the highest total capacity seen during the filtered time range, or an amount that you set based on the service level agreement (SLA) with your ISP.
When provisioned capacity isn’t set, the yellow line on your capacity chart is the capacity of bottleneck, which is either the result of a physical or configured limit. The high point will change a bit based on the filtered time range, but generally it should stay steady. If you notice a change, this could be the result of a route change or a configuration change by an ISP.
Leaving provisioned capacity unset is the approach you should use when end-to-end performance isn’t guaranteed by your SLA. While the link speed to your ISP might be guaranteed, everything on the public internet is best-effort. Generally what you buy from the ISP does end up correlating to end-to-end performance, but you’re not justified in being too strict about your expected level of service.
When you’re monitoring a network circuit that does have a guaranteed end-to-end service level, this is when you should set provisioned capacity. Under the best circumstances, the line would be in the same place as if you had let it float, but it is rightfully fixed, and total capacity should never dip below provisioned capacity for long periods. If it does, read this section on low capacity.