Erwang Sanke (二王三恪) or "the two crownings and the three respects" is something Chinese emperors did to honor the previous dynasty. For example the Sui dynasty came directly before the Tang. So the Tang may honor descendants of the Sui royal family. The Tang would do this by giving these Sui descendants land to rule over. The Tang said you can control these lands and use laws of the previous Sui dynasty if you wish. There may also be a sacrifice at a religious place.