Unknown writing

Titan Writing is an unofficial title used to refer to the yet unnamed and unintelligible writing or code on various items throughout the series that can be linked to the existence of Titans. So far, the series has provided very little information on its origin, use, or true nature, but it is known that the writing can be read by at least Ymir, though it is possible there are more characters capable of reading it.

Form
Titan Writing appears as block-like code but contains some inconsistencies in its multiple appearances. In its first appearance in which the series directly reveals its existence, though still nameless currently, the writing appears partially almost like Japanese hiragana or katakana. Upon closer look, the characters are partially covered in shadow but one clear character can be seen, appearing like the letter Z or hiragana's ko syllable. In later appearances, the writing up close looks noticeably different from the writing found on the can of herring, but from a distance it still appears as writing in indecipherable characters.

Appearances
During Ymir and Reiner Braun's conversation in Castle Utgard, Ymir reads the mysterious writing on a can, which Reiner brings to her attention. However, the conversation is cut short by the pending attack of Titans on the castle. Reiner states that he cannot read the writing at all. Perhaps suggesting the 'warriors' from outside the Walls, Annie Leonhart, Bertolt Hoover, and Marcel, cannot read the writing as well.

The writing also appears on the syringes of Titan-Shifting inducing fluids, used by Rod Reiss. Here, the code appears block-like up close.

Speculation
Due to its repeated occurrences, mysterious dialogue, and involvement with mysterious characters, it can be deduced that the writing is of some importance to the plot as well as the origins of Titans. Ymir and Rod Reiss have been seen having direct involvement with the writing, and its appearance on the syringe that turns humans into Titans hints that the language plays a role in the history of Titans.

Ymir has stated before that she stole the Titan-shifting power from the group of Titan shifters from outside the walls, linking Ymir to members of the Homeland. This means that at some point in the past, likely when Ymir was outside the walls as a Titan, she learned to read Titan Writing. It is possible, then, that Reiner, Annie, Bertolt, and Marcel are able to read as well.

It is unknown if Rod Reiss can read Titan Writing, but its appearance on the syringe suggests that Titan Writing is known and used by the Reiss family. Due to the vast knowledge of the Reiss family on the properties and origins of Titans, it can be further concluded that the writing is definitively related to the secrets of Titans.