The distinction is Rikers is a correction facility for post arraigned prisoners. Ideally, anyone in Rikers is serving a minor sentence or waiting for trial after not being able to make bail, or legit warrants for skipping bail or violating parole. Anyone just scooped off the street is supposed to be held in criminal court central booking aka “downtown”. No one should be held there more than 2-3 days under normal circumstances, for the purpose of deciding whether bail is appropriate. Rikers wait is in the months and years to determine guilt or sentence.