Hi @rasa, the publication you referenced was not produced by Tobii, so I can only speculate but it seems that when they mention:
At more than 5 degrees away from the center of the monitor, accuracy and precision worsen to < 0.6◦, and< 0.25◦ respectively.
I would interpret that to mean that the angle between the screen and the imaginary line leaving the screen to the front of the users’ head.
For example, if you were looking straight ahead with your eyes in the middle of the screen, this angle would be around 0, but if you were to look at the screen non directly (imagine facing directly the far-left side of the monitor whilst looking toward the centre) then this angle would increase…hopefully I have made it clear.
In terms of what the ‘error angle’ signifies, I would take this to refer to the maximum angle of difference between the intended object of fixation (say an ‘x’ on screen) and where the gaze data is actually reported.
Whilst not specific to the EyeX, we have produced a number of documents that explain these concepts in greater details.
You can start here @ https://www.tobiipro.com/learn-and-support/learn/eye-tracking-essentials/what-affects-the-accuracy-and-precision-of-an-eye-tracker/ for an introduction and therafter can browse the whitepaper @ https://www.tobiipro.com/siteassets/tobii-pro/learn-and-support/use/what-affects-the-performance-of-an-eye-tracker/tobii-test-specifications-accuracy-and-precision-test-method.pdf/?v=2.1.1
Please let us know if we can be of any further assistance.