The following is an excerpt from CTH explanation. I have seen better, but this should work. In another source, they actually gave the name of the other unit doing the criminal part. I would have to dig deep to find. Am bad at bookmarking and filing. Just read and keep it all upstairs between the ears.
Additionally, the OIG (Horowitz) would want to keep the testimony of Page, Strzok, Ohr, Baker and Priestap away from the Democrat politicians, well known leakers, within the House Intelligence Committee (ie. Eric Swalwell and Adam Schiff) until he was certain their usefulness as witnesses was exhausted.
The reason for this is transparently simple. The OIG is a division inside the Department of Justice. During an internal investigation if the IG becomes aware of unlawful activity he/she is obligated to inform the AG (Sessions) or AAG (Rosenstein). He can’t ignore it and he cannot delay notification of it. Unlawful activity must be reported.
The IG does not have legal or prosecutorial authority – the IG must immediately refer unlawful activity to the proper authority; essentially to his boss. A DOJ prosecutor is then assigned to work with the IG and essentially creates a parallel investigation focused only on the law-breaking part. [That prosecutor could, likely would, then begin a Grand Jury proceeding; no-one outside the AG, AAG, and that prosecutor’s office would know.]
The prior testimony/statements to the IG by the fab-five would explain why AAG Rod Rosenstein was negotiating with Devin Nunes, and why Nunes came away from those negotiations with wind in his investigative sails.
My nitpick is that the prosecutor doesn't have to "reinvestigate" or "parallel investigate." Like any police/prosecutor hand off, the prosecutor relies on the investigator for most evidence.
As I mentioned in my previous, not only is OIG limited in terms of power (investigation only), he is also limited in terms of target (DOJ only, for Horowitz). And in range of target, he is limited in power to compel testimony, being restricted to government employees - once out, also out of OIG reach.
I totally agree that assignment (the word I would use in conjunction with an inside prosecutor) or appointment (the word Sessions used, and the word used in conjunction with Special Counsel) of a prosecutor is absolutely essential. Signs are that this investigator (Horowitz) is already partnered with a prosecutorial team.