#6 | POSTED BY ET_AL
More on the topic from a 2016 article in Time:
How does the process work, and why does it exist at all?
The process is flexible, and the history is borne of geopolitical caution, according to new reports, public information and interviews with David Priess, who served as an intelligence officer during the Bill Clinton and George W. Bush administrations, as well as other intelligence officials who wished not to be named.
Depending on candidate interest, the briefing could be a one-time event or multiple briefings, up through the November election.
But Priess cautioned against downplaying their impact. "We shouldn't delude ourselves into describing it as a general, generic briefing," he said. "Let's not look over the fact that it is a top-secret, classified briefing."
What about a third-party candidate?
The briefings are a courtesy, with no statute specifying who may receive them. This year, in particular, it's possible third-party candidates could be included, Priess said, noting that the two-party system has fallen out of favor with many voters this cycle.
time.com