I agree DL is completely the wrong term for a leaflet. You are probably right Peter that the above is a description of a landscape 210x99mm leaflet. I doesn't help when people don't know landscape from portrait!
...and it gets worse when you start talking about 'pages' and 'leaves' when doing multi-page work for non-print-knowledgeable clients.... I always triple check how pages/sides and leaves I am having to quote on!