Tested in 1.9.4, dot as decimal point as usual.
The requirement for secondary officers (chief engineers, CIC officer and such) seems to be very restrictive (limited to 1 rank).
This is the naval ranks I have (game default):
This is the ship in question, before 'Senior C.O.' is checked:
Aachen class Destroyer 36,000 tons 1,025 Crew 16,155.2 BP TCS 720 TH 14,400 EM 0
20000 km/s Armour 6-97 Shields 0-0 HTK 208 Sensors 0/0/0/0 DCR 62 PPV 121.48
Maint Life 2.15 Years MSP 8,975 AFR 324% IFR 4.5% 1YR 2,594 5YR 38,903 Max Repair 3000 MSP
Magazine 865.6
Captain Control Rating 3 BRG ENG CIC
Intended Deployment Time: 24 months Morale Check Required
A vice admiral is assigned to the ship as its commanding officer. Tested with rear admiral/captain as commanding officers too.
In this case, the ranks of officers allowed to take the tactical officer's role is only the lieutenant commanders:
Nothing else between lieutenant commanders and captains work.
If 'Senior C.O.' on the ship design screen is checked, the min rank for the commanding officer changes to Commodore:
Aachen class Destroyer 36,000 tons 1,025 Crew 16,155.2 BP TCS 720 TH 14,400 EM 0
20000 km/s Armour 6-97 Shields 0-0 HTK 208 Sensors 0/0/0/0 DCR 62 PPV 121.48
Maint Life 2.15 Years MSP 8,975 AFR 324% IFR 4.5% 1YR 2,594 5YR 38,903 Max Repair 3000 MSP
Magazine 865.6
Commodore Control Rating 3 BRG ENG CIC
Intended Deployment Time: 24 months Morale Check Required
In this case, only commanders are allowed in tactical officer's position. Nothing below (expected), nothing above.
It seems so far only 1 exact rank can be assigned as secondary officers in both cases ('Senior C.O.' checked or not). Not sure if it is intended, or a bug.