What is facultative obligation?

Article 1206 of the Civil Code discusses the concept of facultative obligation, viz.:

ART. 1206. When only one prestation has been agreed upon, but the obligor may render another in substitution, the obligation is called facultative.  

The loss or deterioration of the thing intended as a substitute, through the negligence of the obligor, does not render him liable. But once the substitution has been made, the obligor is liable for the loss of the substitute on account of his delay, negligence or fraud. (n)

A facultative obligation refers to only one prestation agreed upon, but the obligor may render another in substitution. For example, I will give you my phone but I may give you my laptop as a substitute. In this obligation, only the phone is due, Hence, the lost of the laptop will not render the obligor liable. However, if the substitution has been made, the laptop becomes due and demandable. The loss of the laptop will render the obligor liable.