Top soil should be exactly what it says. basically the top 12 or so inches of virgin soil.
Top soil is now sols in a multitude of formats - not all conform to the spirit of the word!
It would be a food idea where possible, to visit the topsoil supplier and see exactly what is being served up.
Some use re-constituted waste soil - which is then screen to give is a semblance of being proper topsoil. The top soil you get should resemble that which is normally available in your geographical area. If you live in Kent - and are serves up with a rich organic peaty soil, it could well be that it is imported from another county.
To go with this, sometimes the only way to get a decent topsoil is to ensure that it 'does' come from another county - or from one of the creditable national distributers.
UK Topsoil - Top Soil Suppliers
Avon Bristol Topsoil
USA Topsoil - Top Soil Suppliers
Alabama Topsoil Supplies
By David Hughes