“This new naming convention is designed to make things easier for users, so they can quickly and easily work out where a device sits within the series and beyond that have a clearer idea of what each series does. Nseries remains the flagship and most advanced range of products. Xseries comes next and focuses on social entertainment. Eseries remains focussed on productivity and business whilst Cseries represents the core range of products.”
There are four series of devices in total and the numeric digits accompanying each handset represent the range of features and price level on a scale of 1 to 9, 1 being the lowest and 9 the highest.
I might say this would be a great help when choosing the best phone that fits your need. From what I know, this same idea has been implemented by Intel with their processors. Intel has moved its new processors to Core-iX naming (X is the core number) to lessen consumer confusions.