Akhyati vada of Prabhakara school is different from Satkhyati Vada of Shri Ramanujacharya.
You have to take a single example of silver in the nacre or snake in the rope example and look at all the classifications of error.
Akhyātivāda of Prabhakara Mimansa. The error lies not in perception but in the lack of appropriate discrimination. Advaita rejects this because silver is perceived here and now. It is not from memory.
sat khyAti – this is the theory of vishiShTAdvaita (Ramanuja) and argues that there must be real silver present in the nacre for one to see. Since all objects are fundamentally made of the same five elements, everything is present in everything else. Hence, perception of silver in the nacre is due to the presence of real silver there. This can be dismissed because then every object consists of everything else.
The advaita view is as follows:
Anirvacanīya-khyātivāda. the illusory object of the snake is a product of ignorance (avidyā) about the substratum of the rope and the error is caused due to Maya (which is also indescribable). In the silver/nacre example, the realities are relative. For example, the silver that I saw does not come under the category of asat or unreal since it is experienced as I saw it. When it is negated by the knowledge that it is nacre, the ‘silver that I saw’ is recognized as not real. Therefore, it is mithyA in relation to nacre. Mithya is Anirvacanīya. As Madhusudhana saraswati says in advaita siddhi: The nature of mithyA is that it is different from sat, and asat. It is ‘sat asat anadhikaraNatvarUpam anirvachanIyatvam’ – its inexplicability arises since it is based on neither existence nor non-existence.
From vedanta paribhasa
Objection: Nevertheless, the mental state or impression in the form of
‘this’, and the state of ignorance of ‘Brahman’ due to nescience are
two distinct entities. According to prabhAkara (who was a student of
kumArila bhatta, but formed his own school of pUrva mImAMsA different
from his teacher), one should treat the perception of the object as
‘this’ and the recognition of the object as ‘silver’, as two separate
processes. I.e. cognition and recognition are two distinct processes
and should not be combined. However, in the perception of illusory
silver, the perception of ‘this’ is getting mixed up with the
recognition of real silver perceived elsewhere so that we have a
mixed-up perception of ‘this is (that) silver’. Hence, the illusory
silver, according to prabhAkara, is not an error in perception but an
error in recognition. A qualified perception such as ‘erroneous
perception’ is therefore not permitted and your above analysis
Reply: Not true. We do not accept prabhAkara’s analysis of error. In
our view, error is taking something other than what it is – atasmin
tat buddhi. Nescience also involves taking the substantive Brahman as
other than what it is; as an object this or object that. This is
fundamental in all erroneous perceptions. In the example being
discussed, I am taking the object that I am perceiving as ‘this’ as
silver instead of what it actually is – nacre. This error is at the
vyAvahArika level. There is an error at the pAramArthika level too.
But the cause for both errors is the same. One and the same
consciousness is reflected in two states involving real (the existence
as object) and unreal (the experiential ‘this’ as a form with
quality). When the real is not recognized due to nescience, the unreal
is taken to be a real object. This is admitted as an error in
perception. The illuminating consciousness, sAkshI, illumines the real
and the unreal components – ‘this is’ and ‘silver’. One is the
substantive of ‘is-ness’, and the other the attributive content of the
vRRitti in the form of silveriness. Thus, an error arises in every
perception and is accepted by advaita vedAnta where the real is
ignored, and the Adheya, or superimposed attributive knowledge forms
the basis for the substantive knowledge.
Based on rope/snake principle,
(1) asat-khyati: rope being present, there appears the snake which is not present there.
(2) sat-khyati: rope itself looking like snake.
(3) atma-khyati: rope remaining unidentified, the remembrance of snake, formerly seen elsewhere, creates the illusion.
(4) akhyati: totally unreal.
(5) anayatha-khyati: mental image of snake projected and seen as if it were in front of oneself.
(6) anirvachaniya-khyati: inexplicable.