俱舍論頌疏論本 28.100

28.100

無相謂滅四者，無相三摩地，謂緣滅諦四種行相相應等持。涅槃離十相，故名無相，定緣無相，得無相名。言十相者，謂色等五、男女二種、三有為相（除住相也）。

'Signlessness refers to the four of cessation': the samādhi of signlessness refers to the samādhi associated with the four aspects of the truth of cessation. Nirvāṇa is free from the ten signs, hence it is called 'signless'; the concentration takes the signless as its object, so it receives the name 'signlessness'. The 'ten signs' are the five [sense objects] such as form, the two types of male and female, and the three conditioned characteristics (excluding the characteristic of duration).