We're working out what we think - we'll add our view here!
Mountain Equipment says:
An ultralight duvet with a weather-resistant shell for alpinists and trekkers needing extra warmth and a tiny pack size. .
A minimalist, low bulk down jacket that provides essential extra insulation on fast moving days. DRILITE® Loft shell fabric gives impressive weather resistance for added peace of mind in exposed situations and a protective hood extends the Frostline’s range further still.