Special quality requirements may apply to interfaces of your own system and maybe as well to your external systems, e.g. safety, throughput or availability requirements. These so-called “service levels” need to be treated with care, because they may have high implementation and operating expenses and -risks.
You could denote special quality requirements or -goals in your context diagram.
In the example below, you find two markers at external interfaces that indicate that the corresponding interfaces influence these Q-goals.