TS EN 206 assigns a coded exposure class to every concrete element based on what it is actually exposed to. Reading the code correctly is how a spec avoids using the wrong mix for a coastal structure.
How the code is structured
TS EN 206 exposure classes use a letter prefix identifying the type of exposure, X for a specific deterioration risk, followed by a number indicating severity. XC covers carbonation induced corrosion, XD covers chloride from sources other than seawater such as de-icing salts, and XS covers chloride from seawater specifically, with XS1 for airborne salt exposure, XS2 for permanently submerged, and XS3 for the tidal and splash zone, the most severe of the three.
A structure can carry more than one exposure class at once if it faces more than one deterioration risk, and the specification should list every applicable class rather than only the most obvious one.
Why XS3 is the one to watch closely
The tidal and splash zone covered by XS3 experiences repeated wetting and drying cycles combined with direct chloride contact, which is a harsher combination than either permanent submersion or airborne exposure alone, and it demands the tightest mix design and the deepest cover among the three seawater classes.
A structural element that moves between exposure zones as tide level changes, such as a pier pile, needs to be designed for the most severe class it will ever experience, not the class that applies at low tide.
Reading it correctly on a drawing
A structural drawing should state the exposure class explicitly next to each element, not leave it implied by the element's general location, since two elements a few metres apart, one directly splashed and one sheltered, can legitimately carry different classes.