Signless is not a jhana and is not neither perception nor non perception. For example, both SN 40.9 & MN 121 appear to say the signless can be attained after the sphere of neither perception nor non perception.
However, other suttas, such as SN 41.6, repeated in MN 44, say the signless can be attained after emerging from the cessation of perception & feeling.
Therefore, the impression is the foremost signlessness does not occur directly from a furtherance of "tranquilization" (as described in SN 36.11, MN 111, MN 26, etc) after the sphere of neither perception nor non perception. Instead, it appears the foremost 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.
MN 43, AN 6.13, AN 7.56 and particularly MN 121 & AN 6.60 appears to clearly say the signless is not the foremost in liberation; is not emptiness (sunnata); and is not Nibbana.
MN 121 says:
‘Even this signless immersion of the heart is produced by choices and
‘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
AN 6.60 says:
Take the case of another person who, not focusing on any signs, enters
and remains in the signless immersion of the heart.
Idha, panāvuso, ekacco puggalo sabbanimittānaṃ amanasikārā animittaṃ
cetosamādhiṃ upasampajja viharati.
Thinking, ‘I get the signless immersion of the heart!’ they mix
closely with monks, nuns, laymen, and laywomen; with rulers and their
ministers, and with teachers of other paths and their followers.
So ‘lābhimhi animittassa cetosamādhissā’ti saṃsaṭṭho viharati
bhikkhūhi bhikkhunīhi upāsakehi upāsikāhi raññā rājamahāmattehi
As they mix closely, they become intimate and loose, spending time
chatting, and so lust infects their mind.
Tassa saṃsaṭṭhassa vissatthassa pākatassa bhassamanuyuttassa viharato
rāgo cittaṃ anuddhaṃseti.
They reject the training and return to a lesser life.
So rāgānuddhaṃsitena cittena sikkhaṃ paccakkhāya hīnāyāvattati.
Based on the above, it also appears there are varying degrees of signlessness. In other words, it appears the signless is not always dependent upon emerging from the sphere of perception nor non-perception or emerging from the cessation of perception & feeling.
Therefore, when the Mahayana attempts to say the signless is superior to or transcends Nibbana; that Nibbana is merely a sign or concept; that Nibbana is the same as Samsara; obviously the Mahayana are wrong & lost in concepts about imagined non-concepts or lost in signs about imagined non-signs.