There is really no "legal/statutory" standard. It's all ethical or rules based. Appointees routinely renege of "promises made" under oath, in confirmation.
I see no way Sessions would personally involve himself in going after Hillary or the Clinton Foundation. But that should be okay - or else we're in a situation where literally only ONE person in all of DOJ would be willing to go after crimes there.
I kinda like this one
“What is Sessions SPECIFICALLY recused from? Probably from prosecuting his own team - DOJ employees. John Huber ,the U.S. attorney in Utah, can convene a grand jury, issue subpoenas, collect evidence and order witnesses to testify — all the usual powers a federal prosecutor has — as he delves into whether the FBI abused its powers when it sought permission then carried out wiretapping of a Trump campaign figure, or DOJ when it trod too lightly in pursuing questions about Democratic presidential candidate Hillary Clinton. He is in every way a “Special Prosecutor”, except in name only.”
This is a very important point for the "Anti-Sessions" hordes to understand. What else is important, is that Sessions delegates all decision making in these matters to somebody trustworthy. Handing off to a deepie defeats the entire purpose.