From MN 43 we have a basic informations about animitta samadhi:
“Friend, how many conditions are there for the attainment of the signless deliverance of mind?”“Friend, there are two conditions for the attainment of the signless deliverance of mind: non-attention to all signs and attention to the signless element. These are the two conditions for the attainment of the signless deliverance of mind.”
“Friend, how many conditions are there for the persistence of the signless deliverance of mind?”“Friend, there are three conditions for the persistence of the signless deliverance of mind: non-attention to all signs, attention to the signless element, and the prior determination [of its duration]. These are the three conditions for the persistence of the signless deliverance of mind.”
“Friend, how many conditions are there for emergence from the signless deliverance of mind?”“Friend, there are two conditions for emergence from the signless deliverance of mind: attention to all signs and non-attention to the signless element. These are the two conditions for emergence from the signless deliverance of mind.”
Later we are informed that unshakeable deliverance of mind is an ideal deliverance most certainly synonymous with nibbana since it has qualities of immeasurable deliverance of mind, the deliverance of mind through nothingness, the deliverance of mind through voidness, and the signless deliverance of mind. None of these separately stands for nibbana. The deliverance of mind through voidness is not synonymous with entering upon and abiding in pure, supreme, unsurpassed voidness, from description provided in the Sutta we can be certain only that it may lead to abandoning of attavada; conceit “I am” isnt mention in it.
In MN 121 animitta samadhi is mentioned instead of perception and feeling, after all the eight attainments but it cannot be synonymous with it since in AN VI :60 Ven Citta Hatthisāriputta is reported to have access to animitta samadhi, nevertheless later disrobed due to sensual desire, thing which is impossible for one who at least one time attained cessation of perception and feeling.
In sutta AN VII 56 which mentions regular types of sekha, ‘one with a residue remaining’: body witness, one attained to view … (5) one liberated by faith … (6) a Dhamma follower, Lord Buddha to that list given by the brahmā Tissa makes addition: “But, Moggallāna, didn’t the brahmā Tissa teach you about the seventh person, the one who dwells in the markless?”
As I see it the cessation of perception and feeling is synonymous the state of arahat after the death of the body, that is to say asankhata dhatu. Animitta samadhi as mentioned cannot stand as synonymous with the cessation of perception and feeling, but it is also is based on direct knowledge of let’s say something which is changelles or deathless element if you wish, but rather of lower quality, since it is for sure accessible by a lower ariyas who aren’t free from sensual desire. In fact ven Ananda proposes it to the Venerable Vaṅgīsa as a one of methods in his struggle against lust “Develop meditation on the signless, and discard the tendency to conceit. … SN VIII 4 But we know from Ven Citta Hatthisāriputta story that it doesn’t guarantee that later mind again cannot be overcome by lust.
While sekkha isn’t arahat, he understands arahat’s as well puthujjana’s experience while puthujjana understands neither. But arahat is synonymous with the knowledge:
“He understands thus: ‘Whatever disturbances there might be dependent on the taint of sensual desire, those are not present here; whatever disturbances there might be dependent on the taint of being, those are not present here; whatever disturbances there might be dependent on the taint of ignorance, those are not present here. There is present only this amount of disturbance, namely, that connected with the six bases that are dependent on this body and conditioned by life.’ He understands: ‘This field of perception is void of the taint of sensual desire; this field of perception is void of the taint of being; this field of perception is void of the taint of ignorance. There is present only this non-voidness, namely, that connected with the six bases that are dependent on this body and conditioned by life.’ Thus he regards it as void of what is not there, but as to what remains there he understands that which is present thus: ‘This is present.’ Thus, Ānanda, this is his genuine, undistorted, pure descent into voidness, supreme and unsurpassed. MN 121
So as I see it right view gives sekha access to changelles, deathles element and provides possibility to direct attention to it, and this is precisely what animitta samadhi is. In other words, unlike jhanas, it isn’t accessible to puthujjana, I also doubt whether puthujjana is able to suspend thinking outside second and higher jhanas for a longer period of time (signlessness excludes active thinking and imagining).