Currently DSP Cookie Matching Partner pixels are fired when a DSP pixel is fired or an impression is served. DSP can fire any partner's pixels against any traffic source.
With GDPR, we know that at least one cookie matching partner has opted to simply block all traffic from EU resulting in their pixel throwing a 403 error on client's sites. Background is available in https://jira.sizmek.com/browse/SE-2325.
By building a capacity in the DSP to control which regions or countries a cookie matching partner may fire in would enable us to rapidly respond to issues such as this.