Bid modifier
A multiplier on the bid when a term matches. Unmatched requests keep the original bid. Not the same list of keys as targeting.
A bid modifier scales the bid when a targeting-like term matches the request. A multiplier of 0 means do not bid on that match. A number between 0 and 10 scales the bid up or down. If the term does not match, the bid is unchanged.
That last sentence is the one that surprises people. A daypart that lists the hours you want will not suppress the other hours. Those hours simply never match, so they keep the full bid. To hold out a window you enumerate the hours to suppress.
Bid-modifier keys are a wider vocabulary than the targeting catalog. Clock keys such as user_time_of_week live here. They are not in the fifty-key targeting list. We were wrong about that for months because we checked one endpoint.
Bid modifiers work on the exchange we buy through. In Adprime Zero they reach buyers through the conversion optimizer, which proposes bidding a wasteful placement down and can apply it for you; there is no free-form bid-modifier editor. They are a graduated alternative to a hard exclude: you can bid a wasteful placement down and keep collecting evidence. A hard exclude ends the experiment.
Read dayparting is not on the targeting catalog for the clock case.