Nothing except a way to distinguish them from other rations in use.
'The actual prototype of the K ration was a pocket ration for paratroopers developed by the Subsistence Research Laboratory (SRL) at the request of the U.S. Army Air Corps (USAAF) early in the war. Two original samples (one version used pemmican biscuits, a peanut bar, raisins, and bouillon paste; the other used pemmican biscuits, a small D ration bar, canned processed meat, and lemon beverage powder) evolved into the one-package breakfast-dinner-supper combination later adopted as standard. The Quartermaster Command's Subsistence Branch altered some components and renamed the ration the Field Ration, Type K, or "K ration"; the final version totaled 2,830 calories a day. The first procurement of K rations was made in May 1942.Although rumor has it that it was named after Dr. Keys or was short for "Kommando" (as elite troops were the first to receive it), the letter "K" was selected because it was phonetically distinct from other letter-name rations.'
They were designed for short-term use, when the more complete A and B rations were not available.
Rumor has it that it was named after Dr. Keys who first designed the meals or was short for "Kommando" (as elite troops were the first to receive it), the letter "K" was selected because it was phonetically distinct from other letter-name rations.
https://en.wikipedia.org/wiki/K-ration