NTI Reader uses Chinese text from the Taishō Shinshū Daizōkyō, which means ‘Taishō Revised Tripiṭaka’ (Takakusu Junjiro 1988). The actual text used was digitized by the CBETA project (Wittern 2006). The Taishō Shinshū Daizōkyō (Taishō) is based on the Korean canon (Lancaster 2004, Introduction). CBETA has made the text freely available under a Creative Commons License (CBETA 2017a, ‘Taishō Tripiṭaka’) for volumes 1-55 of the Taishō, which are the ones used in the NTI Reader. Volume 55 is partial. This includes most of the Chinese content of the Taishō. The remainder is Japanese commentaries, Siddhaṃ texts, and artwork. You can view the entire contents Taishō at the SAT website.
The Korean version of the Chinese Buddhist Canon is based on was printed from blocks that were carved over the period 1236-1251. After the Kaibao canon was taken to Korea, the Chinese canon kept evolving. Each major dynasty, including the Yuan, Ming, and Qing produced their own versions under the sponsorship of the Chinese imperial court (Fo Guang Shan 2000, s.v. ‘中文大藏經’). One of the best known and most widely distributed versions of these was the 龍藏 Lóng Zàng produced in 1738. Production of modern versions continued this tradition in the Republican period and in Taiwan after 1949. Partial digitizations of many of these versions of the Chinese Buddhist canon have been digitized and are available on the CBETA Hanwen Dazangjing (《漢文大藏經》) page. The 佛光大藏經 “Fo Guang Buddhist Canon” was published by Fo Guang Shan in 1977 (Fo Guang Shan 2000, s.v. ‘中文大藏經’). The later Chinese versions of the canon contains texts that are not in the Taishō.
The Taishō introduced a modern numbering system with roughly chronological ordering. This revolutionized the classification system of the canon and replaced the Thousand Character Classic index used by historic versions (Wu 2016, loc. 705). Previous versions of the canon placed Prajñāpāramitā at the beginning of the canon in accordance with Tiantai theory of the Mahāyāna teachings were the ultimate teachings of the Buddha. The Taishō places the Āgamas in accordance with modern scholarly understanding that they probably most closely match the earliest teachings. The Āgamas in volumes 1-2 of the Taishō are closely related to the Pali Nikāya.
There are 2,239 Chinese texts included in the NTI Reader. The Korean version that the Taishō is based on was printed from blocks that were carved over the period 1236-1251 and includes 1,512 titles (Lancaster 2004, Introduction). There are some Chinese texts in the Taishō that come from later periods.
The development of East Asian Buddhist canons includes extensive historic scholarly work in compiling, cataloging and investigation of textual authority (Wu and Chia 2016, loc. 297-447). Early catalogs, such as the Collection of Records Concerning the Chinese Buddhist Canon 《出三藏記集》 (T 2125), are themselves included in the Taishō in Volume 55.