Why is the third stage of the calvin cycle called the regeneration stage?

Explanation:

Carbon fixation, reduction and regeneration are the three stages of Calvin cycle. During reduction reactions six molecules of glyceraldehyde-3-phosphate (G3P) are formed. Out of which one G3P leaves Calvin cycle to produce glucose and the remaining five G3P molecules continued in the cycle. They are used to regenerate ribulose bisphosphate (RuBP) to fix more cabon dioxide.  In regeneration of RuBP, three more ATP molecules are used.   Thus, to run the Calvin cycle, regular supply of RuBP to fix more carbon dioxide is required, so it is called regeration.  

high growth (apex)