The Enforcement Act of Act of Military Service System Article 6

In accordance with Paragraph 1 of Article 9 or Paragraph 1 of Article 10 of the Act of Military Service System, the one takes a voluntary test for Reserved Ranking Officer Service and Reserved Non Commissioned Officer Service shall be divided into volunteer and compulsory during the Active Service.
中文原文 兵役法施行法第6條
依兵役法第九條第一項或第十條第一項規定,依志願考選服預備軍官役、預備士官役者,於服現役時區分為志願役、義務役。

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