What factor is primarily used for apportioning income in State Z according to the provided information?

The weighted sales factor is the primary method for apportioning income in State Z, making it a significant component of the state's taxation framework. The weighted sales factor allows the state to determine how much of a company's income should be attributed to its activities within the state based on its sales figures. By emphasizing sales, this method often reflects the relative economic presence a business has in the state.

This approach typically involves calculating the ratio of sales made to customers in State Z compared to total sales everywhere. Using this ratio, businesses can effectively allocate their income based on where their economic activity occurs, which can help ensure that tax liability is fairly distributed in accordance with the business's operational footprint.

Significantly, the emphasis on sales in apportioning income aligns with many states' strategies to attract businesses, as it often results in a more favorable tax outcome for companies with substantial sales outside the state. In contrast, methodologies focusing solely on property or payroll might not accurately reflect the income-generating activities that primarily drive a business's success.