Atomic code golf is scored by the number of operations in a specific fragment of a language you define.
Atomic code golf asks you to solve a task using only a limited set of operations, with as few of these operations as possible. Often, these are used to make challenges about bit-bashing, logic gates, and arithmetic circuits. By not allowing more powerful or general-purpose operations, the task focuses on more direct low-level implementation.
In writing a challenge, you need to specify very precisely what operations may be used. This is basically defining a fragment of a language that answers must use. Be careful to say exactly what the operation does: for example, modulus works differently with negative numbers in different languages. Ideally, you could provider an interpreter for the language.