Buying a New String

In Chefland, each citizen has their own personal string. Chef is no exception. Recently, his string broke and he went to a shop to get a new one. There are two strings in the shop: $A$ and $B$. Chef may choose any substring $A_s$ of $A$ (possibly empty) and any substring $B_s$ of $B$ (also possibly empty). Then, the shop assistant cuts these substrings out and sells a single string $C = A_s + B