A denomination is a proper description of a currency amount. It is oftentimes a sub-unit of the currency. For example, traditionally a cent is 1/100th of a particular unit of currency.).
Haven denomination names add SI prefixes after dropping the initial “mo” for ease of use. Actually, the smallest unit of Haven is 1 piconero (0.000000000001 XMR).
Support for input using SI prefixes was added to the Haven codebase on March 3, 2017 by Havenmooo. The smallest unit of Haven (10^-12 XMR) was originally called a tacoshi in honor of user Tacotime, an early Haven contributor and was later renamed for ease of use and consistancy.
The smallest fraction of Haven in the current havend implementation is also known as the @atomic-unit, which is currently one piconero.