Patisambidhamagga explains that contemplation on voidness or emptiness is the same as contemplation on anatta. Anattanupassana=sunnatanupassana. (treatise on insight, patisambhidamagga)
What does it mean the world is empty or void? The eye and visable form, the nose and odours…the mind and mental objects…and also any feeling that arises due this 6 sense-contacts is empty of a self or anything belonging to self or empty of anything everlasting, permanent, eternal and not subject to change. (Psm, Treatise on Voidness, §1/3). It seems to say that the senses and all they sense is subject to change, there is nothing everlasting in it, not a self. I think some sutta’s also say, all reactions based on feelings due to sense-contact are empty.
According Treatise on Insight (§9)
Contemplating Anatta means seeing (formations) as: alien, as empty, as void, as vain, as not-self.
It also mentions vain (fruitless, of no use).
Seeing conditioned phenomena as: giving no shelter, no refuge, no protection belong to dukkha nupassana. As also; seeing formations as painful, disease, boil, dart, calamity, affliction, plague, disaster, terror, menace, the root of calamity, danger, murderous, subject to cankers, Mara’s materialistic bait, connected with the idea of birth, aging and sickness, sorrow, lamentation, dispair, and as connected with the idea of defilement.
This is related to seeing that the cessation of the 5 aggregates (or 6 sense domains) is Nibbana and is described as pemanent. (ToI§8)
This means seeing conditioned phenomena as: impermanent, desintegrating, persishable, unenduring, subject to change, having no core, due to be annihilated, as formed, connected with the idea of death.
From this perspective, practically speaking, in applying the Dhamma, anicca, dukkha and anatta have not 1 fixed rigid meaning. It seems to me it is more about seeing the general picture, the idea.
A very strong and direct meaning of anatta is, i find, is the idea of alieness. This idea is very direct connected with the idea that for example, thoughts, or emotions, is not me and does not belong to me. Alien. Buddha uses in the sutta’s a nice example of people who see others accumulating wood for a fire. Do they think?: 'oh wee, now they are going to burn me!"…ofcourse not. You are not that wood. Likewise for the khandha’s. That idea of alien is, i feel very powerful.