Signless is not a jhana and is not neither perception nor non perception. For example, both <a href="https://suttacentral.net/sn40.9/en/sujato">SN 40.9</a> & <a href="https://suttacentral.net/mn121/en/sujato#11.1">MN 121</a> appear to say the signless can be attained after the sphere of  neither perception nor non perception.

However, other suttas, such as <a href="https://suttacentral.net/sn41.6/en/sujato#8.2">SN 41.6</a>, repeated in MN 44, say the signless can be attained after emerging from the cessation of perception & feeling.

Therefore, the impression is the foremost signless does not occur directly from a furtherance of "tranquilization" (as described in <a href="https://suttacentral.net/sn36.11/en/bodhi">SN 36.11</a>, MN 111, MN 26, etc) after the sphere of  neither perception nor non perception. Instead, it appears the signless occurs after emerging from  the sphere of  neither perception nor non perception; or otherwise, emerging from the cessation of perception & feeling; when the mind becomes increasing conscious rather than less conscious. 

In conclusion, MN 43 and particularly <a href="https://suttacentral.net/mn121/en/sujato#11.1">MN 121</a> appears to clearly say the signless is not the foremost in liberation; is not emptiness (*sunnata*); and is not Nibbana. <a href="https://suttacentral.net/mn121/en/sujato#11.1">MN 121</a> says: 

> ‘Even this signless immersion of the heart is produced by choices and
> intentions.’
> 
> ‘ayampi kho animitto cetosamādhi abhisaṅkhato abhisañcetayito’.
> 
> They understand: ‘But whatever is produced by choices and intentions
> is impermanent and liable to cessation.’
> 
> ‘Yaṃ kho pana kiñci abhisaṅkhataṃ abhisañcetayitaṃ tadaniccaṃ
> nirodhadhamman’ti pajānāti.

Therefore, when the Mahayana attempts to say the signless is superior to Nibbana; that Nibbana is merely a sign or concept; that Nibbana is the same as Samsara; obviously the Mahayana are wrong.