By what the person will actually spend their time doing in the United States, supported by an organisation chart showing who performs the operational work. Job titles and seniority do not settle it.
Answer a proportion question with a proportion
Hypothetical example: the safety director's American week is projected as roughly two-thirds setting policy, reviewing audit results, directing three staff and reporting to the owner, and one-third reviewing files personally during the integration. Presenting that split, with the chart showing who handles daily compliance work, addresses the requirement directly. Presenting a list of twenty responsibilities does not.
Note that supervising a small number of non-professional employees is treated differently from managing an essential function, so if the function reading is the accurate one, describe the authority: what the director approves, what standards they set, and what they can stop. All the other requirements still apply — a qualifying corporate relationship with both entities doing business, one continuous year of qualifying employment abroad within the last three years, and a maximum of seven years in L-1A status counting earlier U.S. periods.