Thanks for your responses.
In Australia, "battens" are used both for tiled rooves and metal rooves (as "roofing battens"). With the metal rooves they would be at 450 or 600mm spacings. Battens morph into purlins, when the rafter spacing spreads perhaps beyond ~1 to 1.5M, and the between rafter span requires a larger cross section dimension. Battens are also often used to carry the internal ceiling plaster, instead of attaching to the ceiling structural members. If I was building a shed with portal frames or trusses spaced at 4M I would use "purlins"
So without the battens listed on the roof, the sheeting will be shown in fact to be around 35 to 50mm lower than it actually is, which might catch you out with the sills when fitting dormer windows etc. The same applies to the ceiling plaster (or dry wall as you call it). The ceiling height will be 25 to 35mm lower in reality.
Anyway, you have answered my query promptly, so thanks. I will put the information into the mix.