From the ACEA publication "ACEA European Oil Sequences":
Gasoline Sequences
A1 Oil intended for use in gasoline engines specifically designed to be capable of using low friction, low viscosity oils with a High Temperature / High Shear Rate Viscosity of 2.6 to 3.5 mPa.s. These oils may be unsuitable for use in some engines. Consult owner manual or handbook if in doubt.
A2 General purpose oil intended for use in most gasoline engines with normal drain intervals, although it may not be suitable for some high performance engines.
A3 Stable, stay-in-grade oil intended for use in high performance gasoline engines and / or for extended drain intervals where specified by the engine manufacturer, and / or for year-round use of low viscosity oils, and/or for severe operating conditions as defined by the engine manufacturer.
A4 Reserved for future use for gasoline direct injection engines.
A5 Stable, stay-in-grade oil intended for use at extended drain intervals in high performance gasoline engines designed to be capable of using low friction, low viscosity oils with a HT/HS of 2.9 to 3.5 mPa.s. These oils may be unsuitable for use in some engines. Consult owner manual or handbook if in doubt.
Light duty diesel Sequences
B1 Oil intended for use in car and light van diesel engines specifically designed to be capable of using low friction, low viscosity oils with a High temperature / High shear rate viscosity of 2.6 to 3.5 mPa.s. These oils may be unsuitable for use in some engines. Consult owner manual or handbook if in doubt.
B2 General purpose oil intended for use in most car and light van diesel engines (primarily indirect injection) with normal drain intervals, although it may not be suitable for some high performance engines.
B3 Stable, stay-in-grade oil intended for use in high performance car and light van diesel engines and / or for extended drain intervals where specified by the engine manufacturer, and / or for year-round use of low viscosity oils, and/or for severe operating conditions as defined by the engine manufacturer.
B4 Stable, stay-in-grade oils intended for use in cars and light vans having direct injection diesel engines but also suitable for applications described under B3.
B5 Stable, stay-in-grade oil intended for use at extended drain intervals in car and light van diesel engines designed to be capable of using low friction, low viscosity oils with a HT/HS of 2.9 to 3.5 mPa.s. These oils may be unsuitable for use in some engines. Consult owner manual or handbook if in doubt.