The test HMRC applies is function, not shape. It accepts that items printed on stiff paper and card are not automatically excluded from the leaflet meaning, but treats the stock as an indicator: if the item's main function is to be kept or used for a specific purpose in its own right, rather than being ancillary to another supply, it is not a leaflet. A door hanger that a guest turns round for months is precisely that, and the same reasoning puts calendars, admission tickets, discount vouchers and reference cards outside the zero rate.
What decides it?
It is kept and used. That is the whole reason it is not a leaflet.
Where does this come from?
HMRC VAT Notice 701/10, paragraph 3.3, HMRC's guidance on zero rating books and printed matter. See the notices we work from.
Some real jobs
Do not disturb hangers on 400gsm board for a hotel chain20%
Kept and used for a purpose of its own.
The same shape printed as a thin promotional flyer for a door drop0%
Same outline, different function. Read once and thrown away.