A famous passage of Xenophon goes as follows (Xen. Hell. 1.4.3):
Cyrus had a letter with him, bearing the King’s seal … among other things it contained these words: I am sending Cyrus down to the coast as karanos of all whose mustering centre is Castolus (the word karanos means „having power“)
Xenophon never repeats the word karanos, and no other surviving Greek or Latin writer uses it. In the Anabasis (1.1, 1.9.7) he says that Cyrus was made strategos, or general, of those whose mustering centre is Castolus.
The word karanos has become encrusted with a painstaking and scholarly literature which investigates it philologically. Because the term was only attested once before the Parthian period, when it appears in Aramaic on coins and is spelled krny and equated with Greek autokrator, progress has been limited. The term clearly contains the root kāra-, the Old Persian word for the politically and military significant part of the population. This word is not easily translatable into English, but there are convenient equivalents in many languages, including German Heeresvolk. Because it appears in both the royal inscriptions and in Iranian names, its general meaning is clear. Philologists disagree whether the ending /-nos/ is simply the suffix for „someone in charge of“ (Latin tribus -> tribunus) or from a verb “to lead, to make go” as Nicholas Sekunda prefers (Gr. στρατηγός <- stratos “army” + agō “to go”, δημαγωγός <- dēmos “people-in-arms” + agōgos “one who leads astray”). In the first case the Old Persian would be something like kārana-, in the second kāranaya-. Neither theory clarifies exactly what the word meant in 407 BCE. Scholars who attempt to show that karanos was a common title in the Achaemenid empire find themselves in a foggy jungle, since just because a karanos could be called a strategos does not mean that any of the other strategoi in Greek sources were karanoi, and the masses of Elamite, Babylonian, Aramaic, and Demotic Egyptian documents did not use this term. But then the group of leather documents from Bactria from the fourth century BCE was published, and many of its readers noticed something.
One of the letters contains a date followed by wštˀsp krny, which a Greek would have pronounced as Hystaspes karanos (the Greeks whose dialect defined our spelling conventions had trouble pronouncing /v/ and /š/, so Persian Vishtaspa became Greek Hystaspes). Modifiers usually come after the noun that they refer to in Aramaic, so this looks very much like a title, and the spelling matches the word on Parthian coins. All of a sudden, we have an example of this word outside of Xenophon, and from the opposite end of the empire no less. And it has the /y/ at the end which Sekunda suggested the original Persian word should have.
Thus these leather documents have doubled our attestation of karanos/krny in the Achaemenid period. They give us some reason to prefer Sekunda’s +kāranaya- “leader of the kāra” over the traditional +kārana- “kāra-er” as the original Iranian form which lies behind the Aramaic and Greek. And they give us some reason to think that this was a recognized title, and not a special appointment for an unusual situation. Sometimes in ancient history, a little evidence can make a big difference.
Further Reading: John Hyland, “Vishtashpa krny: An Achaemenid Military Official in 4th-century Bactria,” Arta 2013.002 (link), Eduard Rung, “Some Notes on Karanos in the Achaemenid Empire,” Iranica Antiqua 50 (2015) pp. 333-356, Nicholas V. Sekunda, “Achaemenid Military Terminology,” Archäologische Mitteilungen aus Iran 21 (1988) §6 p. 74
Edit 2016-05-10: Added citation to Sekunda’s suggestion.