Let's say here, the res essentia, the essence or conceptual reality, is that of a 'gold ring'. That it is 'gold' and 'a ring' are its real predicates and these cannot be changed without altering its essence. If you do remove 'gold' you are simply left with 'ring', and if you remove 'gold' and 'ring' you are left with nothing.
Conversely, accidens is different.
something that befalls or is added to the being - including its actuality - is an accidens
Actuality is something accessory to the what of a being
everything that is not a real predicate of a res, must be caused
An example of accidens would be if someone were to make the gold ring. This predicate could be removed, so to speak, by not making the ring. The essence of the ring would be unaltered. Another accidens could be for the produced ring to be lost. This predicate could be removed by finding the ring. Again, the essential concept, "gold ring", is unaltered throughout.
Referenced material: St Thomas Aquinas' interpretation of Aristotle's ontology :-
The problem of the relationship between essence and existence is
resolved in the Thomistic school by saying that in an actual being the
what of this being is a second res, something else for itself as over
against the actuality; thus, in an actual being we have the
combination or composition, compositio, of two realities, essentia
and existentia. Therefore, the difference between essence and
existence is a distinctio realis. Cum omne quod est praeter
essentiam rei, dicatur accidens; esse quod pertinet ad quaestionem an
est, est accidens; since everything that [in the Kantian sense] is not a real predicate in a being is spoken of as something that befalls
or is added to the being [accidens], to the what, therefore the
actuality, or existence, that relates to the question whether a res
with the totality of its realities exists, is an accidens. Actuality
is something accessory to the what of a being. Accidens dicitur
large omne quod non est pars essentiae; et sic est esse [that is,
existere] in rebus creatis; existence is not part of the reality but
is added on to it. Quidquid est in aliquo, quod est praeter essentiam
ejus, oportet esse causatum; everything that is outside the
thing-content of a thing, everything that is not a real predicate of
a res, must be caused, and indeed vel a principiis essentia . . .
vel ab aliquo exteriori, either by reason of the essence itself or by
source :- The distinction between essentia and existentia in Scholasticism
Rev. Joseph Owens maintains that the Thomist interpretation differs from Aristotelian ontology in its treatment of actuality as accidens (viz. below). However, a commentary by Heidegger, here : - Does existence precede quiddity? - confirms the Thomist interpretation.
source :- The Collected Papers of Joseph Owens