Δt refers to the difference in temperature between the water circulating in the central heating system and that of the ambient temperature. It is important to use the correct Δt when selecting your radiators, as the same radiator will have different outputs at different water temperatures due to the heat source.
Δt50 is the UK standard for domestic gas boilers, however Stelrad also quotes at Δt30 and Δt40 for lower water temperature systems, such as renewable systems.