According to bookseller/publishing industry standards, anything with 49 pages or less is considered a bulletin, brochure or pamphlet.
That is the difference in the strictest sense of the definition. Obviously, clubs and organizations publish periodicals that they refer to as "magazines" which are less than 49 pages. I think the general understanding is that a "bulletin" contains current news and goings-on in the organization, while a magazine might contain not only news, but also "filler" material - a biography of a member, puzzles, things like that.
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