The approach is similar to that used for the selection of biological Sites of Special Scientific Interest (SSSI), but the thresholds are lower.
Selection is primarily for habitats of inherent wildlife interest, but some sites may be selected for supporting rare or scarce species of plants or animals outside such habitats.
The areas concerned may be areas of 'natural' habitats, or they may be man-made – for example, the 40-acre (160,000 m2) West Norwood Cemetery is designated a Site of Metropolitan Importance for Nature Conservation by the London Borough of Lambeth Once identified, designation and protection of the areas are done by local authorities through planning policies in their development plans.
National government guidance (PPS 9) requires all development plans to include such policies.
The variation in names for the designation reflects its separate existence in the different development plans for different areas.