Marginal utility is the extra usefulness or satisfaction a person gets from acquiring or using one more unit of a product

This statement is TRUE.

This is the exact definition of the concept of marginal utility, which is understood as the satisfaction provided by the consumption of one extra unit of a good/service. When an economic concept is defined in the margin it always refers to the outcome generated by an aditional or extra unit of a product, of a cost, of a certain source of income, etc

The marginal utility is used to determine the demand of a product as it is the measurement of the satisfaction gained from the aditional unit, which is also connected to the consideration (by the consumer) of the scarcity or abundance of the product.