No UC code could mean two things: either it was ripped off on purpose, or by accident, or its UC code is so low it doesn't rate to be classified (garbage). UC codes are "usage categories", running from UC1 (the lowest) to UC5 (the highest).
But the UC code is only half of it. A UC code must be matched to the retention rating of the chemical injected into the wood. This retention rating is called PCF (PCF = pounds per cubic foot), and shows how much more a cubic foot of wood will weigh after it's treated,: For example a .25 PCF equates to weighing 1/4 lb heavier per cubic foot.
"Indoor" usage may be in the range of .15 to .20 PCF, which would place it in
and around UC1/UC2. I like to see a PCF number in and around .30 or .40 for outdoor exposure. .60 to .80 PCF is ground contact, and may be classified as Marine Duty (certainly the good stuff)
Very low PCF ratings could be overcome by letting the wood dry out fully and giving it a soak coating of 2/3 varnish mixed with 1/3 mineral spirits (about a 30 to 40% reduction)
The reduction keeps it wet longer to get "soak in". I've gotten good long term results doing this.
I've seen very low PCF numbers (e.g. .02 / .03) which in my opinion is not worth
considering to purchase (garbage).