The seventh visuddhi refers to Sotapatti nana, the first knowledge of the Path - Nanadassana visuddhi. Nana is knowledge, dassana is vision, visuddhi is purification.
When you have attained the seventh visuddhi you have become a Sotapanna Sota means stream; apanna means enterer. Sotapannameans stream enterer. When you have attained Sotapatti nana, you have entered into the current of the Noble Eightfold Path. Only then have you attained Nanadassana visuddhi - purification of knowledge and vision.