Resolution is based on who actually delivers the briefing from the podium, not on who is listed or announced in advance. Katie Zacharia will not qualify if she appears at a briefing only as a guest, or speaks only after being introduced by the individual leading the briefing. For the purposes of this market, a White House press briefing is an event listed on the public White House schedule as a press briefing.
"New Media" briefings and special briefings including additional guests will qualify, so long as Katie Zacharia leads the briefing. Impromptu press gaggles, and remarks that are not listed on the public schedule as a press briefing, will not qualify. If Katie Zacharia jointly leads a qualifying briefing with another individual, the briefing will qualify.