佛祖統紀 3.140

3.140

不定得名。玄文。南北地通用三種教相。一頓。二漸。三不定。又古師立偏方不定。謂非次第。別為一緣。如金光明勝鬘等是也（云云）。知不定之名出於古師。天台更加祕密為四教。於義方足。

The derivation of the name 'Indeterminate': The Xuan text says: 'The northern and southern regions commonly used three doctrinal characteristics: first, Sudden; second, Gradual; third, Indeterminate.' Moreover, ancient masters established the 'regional indeterminate,' meaning it was not sequential, but a separate, specific condition, such as the Suvarṇaprabhāsa, Śrīmālā, and so forth (and so on). Thus we know that the name 'Indeterminate' originated with ancient masters; Tiantai additionally added the Secret to form four teachings, whereupon the doctrinal meaning became fully complete.