3.6N Aircrete blocks. Where are they used?

[flanagaj]

I am just going back through my estimators quote and am trying to workout where the 3.6N Aircrete blocks they have specified are used.  The quantity seems to indicate that they have specified them for the ground and first floor cavity walls and only specified the 7.3N blocks for the internal ground floor blocks walls.

 

Is this normal practice or can you use 7.3N blocks for for everything internally and avoid using the expensive Aircrete blocks?

[ADLIan]

Aircrete blocks in external walls to help achieve Uvalue. Check construction, insulation and required Uvalue. Cheap & cheerful standard concrete block for inner walls, density helps with acoustics.