Cincinnati creates the update graph using the primary metadata (baked in the release image) and secondary metadata . Do make upgrades more successful and protect customers from bugs which get discovered after a release we block edges  to that particular release. The bugs are evaluated to determine the impact on overall fleet of OpenShift clusters. Some bugs only impact specific platforms, or clusters with other specific features.
In those cases there is currently tension between wanting to protect vulnerable clusters by blocking the edge vs. wanting to avoid inconveniencing clusters which we know are not vulnerable and whose administrators may have been planning on taking the recommended update.
For example was SRE wanted to blocked edges into 4.3.19 temporarily for OpenShift dedicated while rhbz#1838007  was investigated but the issue was not seen outside of OSD hence it did not make sense to block the edge for all the clusters.
This feature will help cases like this and also help us contain edge removebal to more accurate and not effect all clusters. Also this will give us ability to expose certain edges to certain clusters if required.