One challenge for the Cabinet Secretary Simon Case in deciding whether a group of people drinking together is a party is that there was something of an evening drinking culture in 10 Downing Street, especially on Friday nights and especially in the press office.
He'll have to begin his adjudication of the propriety of Downing Street parties by deciding whether a group of people routinely drinking at their desks in the office constitutes a breach of lockdown rules.
According to a government source: "The number 10 press guys drink at their desks on a Friday evening... that goes on for hours, but still fielding calls/emails etc, so just got old school Fleet Street vibes."
The point is that Case will be reviewing a series of Downing Street gatherings, which look like unlawful parties. Or at least that is what the education secretary Nadhim Zahawi told me on my show last night.
One of those cited gatherings is when Cleo Watson left her job as an important aide to the prime minister on 27 November, which on one interpretation is an impromptu get together, that perhaps sets a bad example, and on another is a rule-breaking event.
According to sources, here is what happened.
I am told she popped round to the press office to say goodbye, which - according to an official - had the "usual scene of press guys having drinks at their desks with a small group of No 10 staffers."
Apparently, Boris Johnson saw what was happening on his way to his flat and "ducked in and said a few words."
The point is that anyone landing from Mars and sticking their head through the door would have seen what looks like a party. And in the context of the national lockdown rules in effect then, it would have seemed a breach of the rules.
But what will Simon Case make of it?
Here is one distinction between the 27 November gathering and the 18 December one, the one that sparked the resignation of Allegra Stratton after she was seen joshing about it in that film obtained by ITV News of a mock Downing Street press conference - on 18 December there were advance invitations, outside guests, wine and cheese, and party games; there was none of that on 27 November.
So the 18 December gathering seems premeditated, and the one on 27 November was spontaneous.
Is spontaneity a defence?
This is tricky.
In November 2020, socialising between households was prohibited, and only "key" workers were allowed to go to offices at all. So the 27 November get-together will be seen by many as inappropriate.
But Case is likely to feel a conflict of loyalties when adjudicating. Apart from anything else, as Cabinet Secretary since September 2020, he is part of the culture he is investigating. So whether it is appropriate for him to be judge and jury, as requested by the PM, will be seen by some as moot.