Which statement accurately reflects the description of the substantive plan used for projecting costs related to other postemployment benefits (OPEB)?

The correct answer points to the substantive plan's comprehensive nature in terms of how costs related to other postemployment benefits (OPEB) are projected. The substantive plan does not solely reflect the current or historical provisions but encompasses all components relevant to the calculation of OPEB liabilities, including both realized amendments and potential changes that are contemplated.

This is critical because the projections for OPEB costs are influenced by what the employer has communicated about the benefits to employees, which may go beyond formal written documents or agreements. Including contemplated amendments means that any changes an employer intends to implement or is considering, whether communicated formally or informally, will impact the valuation of these benefits. This perspective ensures a more accurate representation of the total OPEB commitment by considering future obligations as they may be perceived by employees.

The other options do not fully capture this scope. Understanding the plan solely as perceived by plan members or participating employers limits the recognition of future changes or informal promises that could significantly affect the overall costs. Therefore, the definition of the substantive plan within the context of OPEB must account for both realized and potential future plan changes, which is aptly reflected in the correct choice.