大般涅槃經義記 4.90

4.90

今此偏說悉曇一章以為半字，半名字本，是義云何？今言半者，以悉曇章顯此經中一切義體而未廣辨，故名為半。依於此義，廣開以為一部之經方名為滿。此半與滿大乘不同，向前〈四相章〉中，小乘為半、大乘為滿，良以是中半亦是大，是故學之，得解此經。即此半字與廣為本，故名字本。

Now, here it specifically speaks of the single Siddhaṃ chart as the half letters, and the half is named the root of letters. What does this mean? In saying 'half' here, it is because the Siddhaṃ chart reveals all the substantive meanings in this sūtra without yet broadly distinguishing them, and is therefore called half. Relying on this meaning, broadly unfolding it to make an entire sūtra is only then called complete. This half and complete differ from that in the Mahāyāna: earlier in the 'Section on the Four Characteristics', Hīnayāna was half and Mahāyāna was complete, precisely because here the half is also Mahāyāna; therefore, studying it, one can understand this sūtra. This very half letter serves as the root for the broad [teaching], and is therefore called the root of letters.