#on a symbolic level I think the ring marks can be read as an alliance between pilot and mecha but also between people themselves

[magnoliawaltz]

So, festum body won’t get ring marks?

The ring marks are a small reflection of the assimilation process that occurs when a human pilots a fafner. So, the elements, whose bodies are mostly made of silicon like the festum, either don't get that aftereffect or regenerate fast enough to a previous state that it becomes negligible to them. That is unclear, as far as I know. The know elements so far are Kazuki, Kouyou, Misao and the two Soushis (apparently). Every element is a bit different, and the two Soushis are also different from each other.

I found a settei that explains what is the status of Soushi's body in Exodus:

皆城総士: 通常は人間の肉体でいるが、同化現象に接するとフェストゥムと同じ体に変貌す。 Minashiro Soushi: He normally has a human body, but when he comes into contact with the assimilation phenomenon, his body transforms into the same as a Festum.

We can presume that he had no ring marks because every time he piloted Nicht and was met with that assimilation, his arms and hand were "festum". The same is not true of Soushi-kun, as he has them, same as Miwa. Has his body "returned to a previous state" because Soushi chose to die as a human? I don't know.

An interesting fact is that in Exodus, Narain also had his life "maintained by the festum" and his arm also became golden when he came in contact with assimilation, but he had the ring marks. Presumably because it happened after he had been a pilot for some time. The word "maintain" is important in both of their cases, I think. And Soushi ended up assimilated himself, so it is not like having a festum body prevents that from happening.

Another character who doesn't have the marks is Maris. He is also human. I think in his case the reason is because he always pilots using the soldato armor from Benon that protects the wearer's whole body. Wouldn't that be an important discovery in the Fafner world? But to receive that armor you have to receive Malespero's blessing first. Maybe in the future he'll disclose that info?

Malespero himself does not have them, as he is a core like Misao. And so does Chaos/Mitsuhiro, who is a puppet/alter-ego of Malespero and whose situation is so weird I can't even begin to explain. How different from an element is he? Maybe after I rewatch Exodus I'll understand. But he does not seem to have marks in The Beyond, although he also developed them in Exodus, otherwise he wouldn't be able to pass as human. I don't have a good screenshot in hand to show.

Meanwhile, in The Beyond we have another interesting case: Seri. She is not an element, as Malespero very didactically pointed and has the ring marks, making her human. But her hair turned white as Sakuya's and she says "she can't die". An influence of Altair, probably. What is she? Unknow to me, but her case and Soushi-kun's may be pointing at something.

I am probably missing some things as I haven't rewatched Exodus since 2019, feel free to correct me.