A requirement can be a downstream of another requirement, effectively making the other requirement an upstream of the first one. Downstream implies impact vulnerability. If a requirement is specified as a downstream of another requirement, and the latter (upstream) is edited, it implies that the former (downstream) may be impacted.
The impact is usually in terms of validation, but it can also be on the specification of the downstream requirement. Therefore, when an upstream requirement is edited, it requires that all requirements that are defined as downstream of that one need to be evaluated for impact.
Requirements can be downstream of other requirements either directly or indirectly. But a requirement cannot simultaneously be a downstream and an upstream of another requirement, directly or indirectly.