What does an empty seat represent in early Buddhist iconography?

In early aniconic Buddhist art (Bharhut, Sanchi, Amaravati, c. 2nd century BCE–1st century CE), the Buddha was never depicted in human form. Specific symbols stood for episodes: the empty seat / vajrasana represented his meditation and enlightenment, the Dharmachakra the First Sermon at Sarnath, the stupa the Mahaparinibbana, and a horse without a rider the Mahabhinishkramana (Great Renunciation). NCERT Class XI An Introduction to Indian Art Chapter 4 lists these symbols. The trap is confusing the empty seat with Mahaparinibbana; the seat denotes meditation under the Bodhi tree.