The two kinds of introductory phrase: (a) the ordinary opening phrase of a sutra— "Thus have I heard"; and (b) specific openings referring to the circumstances in which the sūtra was produced.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

諸經開頭時，有通序和別序。如是我聞等是共通於諸經都有，叫通序，序說本經特殊的緣起，叫別序。

陳義孝編, 竺摩法師鑑定, 《佛學常見辭彙》

（名數）諸經之首，有通序別序，如是我聞等之六成就，通諸經有之，曰通序，本經特殊之緣起曰別序。【參見: 序】

丁福保《佛學大辭典》