Regulations Governing the Issuance of Exchange Traded Notes by Securities Firms Article 13

After a securities firm has obtained effective registration to issue ETNs, except as otherwise provided by the competent authority, it may undertake the procedures to reopen the original issue if, for the 5 business days preceding the registration date, the average ratio of the number of units already issued to the number of units to be issued under the original effective registration is 80 percent or higher.
中文原文 證券商發行指數投資證券處理準則第13條
證券商發行指數投資證券,經申報生效後,除主管機關另有規定外,申報日前五個營業日平均已發行單位數占原申報生效發行單位數之比率達百分之八十以上者,得辦理增額發行。

English translation for reference only; the Chinese text prevails. Not legal advice.