What exactly does the "Longmen Twenty Products" in the Longmen Grottoes mean?

There are more than 3,600 statues and inscriptions in Longmen Grottoes, among which Twenty Statues of Longmen is the most famous, and it is the object that calligraphers diligently seek. Many Buddha statues in Longmen Grottoes are engraved with "statues", and the calligraphy of the statues is very wonderful, especially in the Northern Wei Dynasty, which is vigorous and varied. After the mid-Qing Dynasty, some people selected 20 statues of the Northern Wei Dynasty, such as Stone, Yang Dayan, Wei Lingzang and Sun Qiugong, and spread them. This is the famous "Longmen Twenty Products".