大方廣圓覺修多羅了義經略疏 2.111

2.111

本以發心修證，約斷二障，故成五性；此都不斷，故非五數，亦未發心遇教，故言未熏。若據《楞伽》之文，即當第五無性。

Originally, based on producing the mind [for bodhi] and cultivating realization, the five natures are established in terms of severing the two hindrances; since these are not severed at all, they do not count among the five, nor have they produced the mind and encountered the teachings, hence they are said to be 'not yet perfumed.' According to the text of the Laṅkāvatāra, this corresponds to the fifth, the natureless.