Later when a messenger wanted to bring a message past a guard station, the messenger would prove their authenticity by presenting a segment of bamboo, which could then be matched up with a corresponding segment held by the guard station.
Apparently this practice and specifically the splitting of the bamboo into two pieces for this purpose is mentioned in the ancient Chinese dictionary, the Shuowen (https://en.wikipedia.org/wiki/Shuowen_Jiezi).
Source: Lao Kan, Chü-yen Han-chien k'ao-shih (居延漢簡考釋) (Decipherment and Critical Studies of the Inscriptions on the Wooden Tablets from Chü-yen). Chungking: Academia Sinica, 1943-44. Ts'e 1-4, shih-wen, I, 81b-82b; ts'e 5-6, k'ao-cheng, I, 32a-34a, cited in Written on Bamboo and Silk: The Beginnings of Chinese Books and Inscriptions, by Tsuen-Hsuin Tsien, University of Chicago Press, 1962; in particular see pages 106 and 110.