To partially (and tentatively) answer my own question. The following scheme seems to apply:
VERSION=PREFIX + C2|C9 + VERSION1 + VERSION2 where
* Prefix denotes a specific engine as noted in the table below.
* C9 denotes a development build
* C2 denotes a release build
* VERSION 1 and 2 appear to be major/minor versions (although I was not able to confirm this).
Version prefix Engine code DME App software
R0 Early BX8 MG1CS003 DME8.?
R1 B58 MG1CS003 DME8.6
R4 B58TU MG1CS201 DME8.6T0
F4 S58 MG1CS049 DME8.6.S
H4 S63TU MG1CS202 DME II
(See
https://b58.wiki/wiki/Useful_resources_for_getting_started#BMW_software_versioning_side-note)