One way to hold commissary: somebody higher up hands a job to somebody else and says, from here on, this is yours to run. The person holding that job is a commissary. The authority is borrowed and it has edges — it works only inside the job he was handed. In an army, the job that got handed over most often was food: buy it, keep it, give it out. From there the name slid off the man and onto the counter he stood behind.
The supplied source gives Late Latin commissarius, from commissus, the past participle of committo, to commit or entrust. So the word names an entrusted person, which is exactly the top image here. The source also notes it is a doublet of commissar.
來源給的字源是晚期拉丁文 commissarius，來自 commissus，也就是 committō（委託、交付）的過去分詞。所以這個字字面上就是「被交付任務的人」，跟上面那個畫面剛好吻合。來源另外指出，它和 commissar 是同源的雙生字。
Wiktionary entry for commissary (kaikki extract supplied in the evidence packet, source_ref kaikki:3f6f69a190036ae4:1377811926): From Late Latin commissarius, from commissus, past participle of committo. Doublet of commissar.
Two branches
被交付任務的人
這一組指的是人。字根的意思在這裡還看得很清楚：他手上的權力是別人交下來的。
noun · C2
A person given authority by someone higher up to carry out a duty or hold an office on their behalf — a deputy or commissioner. You meet this use mainly in older writing and in church and legal records, where a bishop's commissary, for example, acts for the bishop in a distant part of the diocese.
受委任的代理官員
機構開給自己人用的地方
軍需官管的是「發放糧食」這件事，而發放要有個地點。名字就從那個人，移到了那個地點，再移到地點裡花的錢。
noun · C1
In American English, a shop on a military base where soldiers and their families buy food and everyday goods, usually more cheaply than at shops outside the base.
軍中福利社（超市）