Sessions supposedly recused himself under 28 CFR Sec.45.2 which is based on a criminal investigation. This was a counterintelligence investigation as per Comey's testimony.

Sessions says that he recused himself, on the advice of career ethics experts at the Justice Department, because he thought this was required by the federal regulation controlling “Disqualification arising from personal or political relationship” (28 CFR Sec. 45.2). But judging from the public testimony that former FBI director James Comey has given about the investigation into Russia’s election-meddling, the regulation did not mandate recusal.

Section 45.2 states that an official is disqualified from “a criminal investigation or prosecution” if he has a personal or political relationship with a “subject of the investigation or prosecution,” or with a person or organization whose interests would be affected by the outcome “of the investigation or prosecution.”