As far as I know, the regular army formations (i.e. brigade, corps, and armies) belong to the same series, which is not split by OOB types. Such as you could have the 1st MG Infantry Brigade, with battalions 1, 2, 3, 4, and 5, then the 2nd Motorized Light Infantry Brigade with battalions 6, 7, 8, 9, and 10.
Then you have the independent units (i.e. the ones you raise without an OHQ initially) which belong to different series for each OOB type. Such as the 1st Independent Recon Battalion, then the 1st Independent MG Battalion, then the 2nd Recon Battalion, then the 1st Light Infantry Regiment, then the 1st and 2nd Light Armour Battalions.
Lastly militia units have a serial number and are split in series depending on which Zone spawned them, but the numbers are only used in the unit's name, since the unit counter will display an M in the upper right corner, standing for Militia.
As for your needs, I would maybe suggest something along the lines of
« Unit serial number
e.g. 8th battalion from the 2nd brigade
or 8th independent regiment unit of a given type ».
Unfortunately I'm failing at finding a terser way to say it.
< Message edited by Jdane -- 7/12/2020 10:20:23 AM >