This is the important bit. Scotrail felt stung by the 385 debacle, and wants a lot more input into the process than just 'go build us some units.'
ScotRail will not be procuring any more new purely diesel multiple units, at any point in the future. That ship has sailed. The BEMUs won't even have a small engine onboard, they're pure 25kvAC EMUs with a large battery pack.
There may be an argument for diesel-battery hybrids for the rural routes, but that is outwith the scope of this procurement.
The aim is for a minimum length of 100m on all the Central Belt services. This is the maximum length of Queen Street platform 1, and Waverley platform 18 is only 109m.