A. When it's not held at lunchtime.
I have noticed a tendency to call training sessions "brown bags". I reckon it is part of the same tendency that causes people to say "scrum" when they mean "team meeting" (I use the word 'team' very loosely here).
First, here's what a brown bag is supposed to be according to wikipedia:
A brown bag seminar, session or lunch is generally a training or information session during a lunch break. "Brown bag" is representative of meals brought along by the attendees, or provided by the host. In the USA, those are often packed in brown paper bags. Brown bag seminars normally run an hour or two.
Brown bags are also described in Linda Rising's book "Fearless Change" as a way of introducing people to new ideas without it taking up official project time.
Now here's what they are not: compulsory sessions to disseminate information that is important for the project. You know the kind of thing: you are expected to attend and it is not held at lunchtime. It is not open the the general project but only to the people that are required to receive the information. It is supplied on a need-to-know basis, i.e. if they need you to know then they will tell you. There is a slideshow that is supposed to act as a substitute for attending if for some reason you cannot attend. I call these sessions 'barf bags'.