Supplementary Planning Guidance (SPG’s) supplementary to the Local Plan system. SPG’s are no longer produced.
Supplementary Planning Documents (SPD’s) supercede production of SPG’s – a name change that dates from the beginnings of the system-tinkering that got us the Local Development Framework that has led to The City Plan (which will over time replace The Local Plan).
Both Supplementary Planning Guidance and Supplementary Planning Documents are used by planning officers with the Local Plan (and in future the City Plan) to judge planning application material.
Umbrella Strategy documents – Local Plans (and The City Plan is one) – are huge undertakings which when adopted provide the overall planning/infrastructure policy framework. They are 20-30 year forward plans so along the way issues arise which necessitates production of these modifiers.