Imperial code cylinders
These indicate the rank and other identifying information of Imperial officers:
Tarkin squinted at the hologram that appeared alongside the
holopresence of the facility administrator. Dressed in an Imperial
uniform, the man was tall and lean, with thick red hair and a raised
scar on his left cheek that ran from the corner of a full mouth to a
bionic eye not unlike the one worn by Vice Admiral Screed.
cylinder identified him as Commander LaSal.”
Code cylinders also allow officers authorization for various activities. For example, some opponents of the Empire used them to obtain fuel:
“A point worth considering,” Tarkin said. “In addition, they’ve
betrayed themselves in other ways. Not only are they conversant with
the Carrion Spike’s instruments, they are also well acquainted with
Imperial procedure. The self-styled commander looks every bit an
officer, and he used code cylinders to requisition the fuel cells.” He
looked up at Vader. “Some of the Empire’s own?”
It seems that the number of code cylinders increased with one’s rank. Tarkin, as Grand Moff, had four:
To complement his new station, he designed and had made a gray-green
uniform whose thick-belted, round-collared tunic featured four code
cylinders and a rank plaque of twelve multicolored squares, six blue
over trios of red and gold. In all dealings with the Emperor he was
referred to as Grand Moff, but for ordinary interactions with military
personnel he retained the honorific Governor.
On the other hand, Admiral Ozzel had three:
All those random officers in the picture given in the question appear to have one or two cylinders.
Thus, there was likely some overlap in code cylinders, with higher-ranking officers having additional cylinders granting additional privileges.