Adding to the muddy the water, the BAe146 / RJ used ‘set’ numbers which became airframe build numbers on assembly.
The aircraft reference consisted of a four number group, the first represented the subtype - length of centre fuselage, 1=100, 2=200, 3=300, and the last three numbers the serial number of the assembled airframe and matching sub’set’ components.
The plan was for the finally assembled aircraft to have consistent sequenced parts after the size - length, was defined by sales; nose from Hatfield, centre fuselage Bristol (the variable bit), and tail section Chadderton. e.g. 3121 was a 146-300 and 121st in ‘build sequence’ (not necessarily as built or flown).
All started well until ‘set’ 19 tail section fell off the back of a lorry on the M60. Thereafter a more general mix and match policy ensued.
The chosen numbering system, the final aircraft identification, was the master certification reference and thus influenced by the ‘home’ regulating agency - UK CAA, who required a reference and tracking system related to the aircraft type approval.
Further complications arose with the RJ because these were built under the umbrella of the 146 type certification, but with modifications applied, - there was no independent type designation. e.g. 2207 was the ‘first’ RJ (85) but built as and amongst the final 146s.
So to address the question, the designation or change of term probably relates to the requirements of certification authority (civil / military) and need to track design, build, and in-service modification.
Also, a generalised MSN system enables additional references, dash no, to cope with different engines on the same aircraft ‘type’, additional systems, operational capability, and customer specific changes which affect certification.