The C series uses Cangjie input codes as the basic sorting mechanism.
What are Cangjie input codes?
Cangjie input codes are a way of inputting Chinese characters based on their shape (or if you like, their elements). With this method you can actually touch Type Chinese characters even if you don't know how they are pronounced.
Because this system maps 24 basic shapes to 24 elements of the English alphabet, it's actually a way of alphabetizing Chinese characters. Sort of. At the very least, the nice thing about using cangjie input codes to lookup characters is that the system is fairly consistent. The first element is always the left most or uppermost or surrounding element.
Using the Easy shape system to index Chinese characters
E series posters are sorted using the Easy shape system. Where sorting by cangjie input code means you have to be familiar with 24 basic shapes (and their derivatives), the easy shape system is based on 12 basic shapes (each with up to 16 derivatives, though this limit may change!).
The easy shape system is designed to specifically to make Chinese character lookup easy.
Indexing Chinese characters via their first or final element
Series 1 and series 3 variations. For both the C and E series there are 1 and 3 suffixes. C1 and E1 are both sorted by the initial shape or element of a character. C3 and E3 are both sorted via the final shape or element of a character.
The benefits of indexing Chinese characters via their final element
Why bother with indexing characters via their final element? In some cases Chinese characters with the same final element have the same pronouncation or similiar pronounciation. So when using the PRS variants, with the C3 or E3 series you can see these characters together which may make it easier for you to learn their pronounciation.
In terms of cangjie input codes, seeing characters with the same final elements grouped together will help you to get a better grasp of how cangjie input codes vary across different types of characters.
Note that the current series of PDF posters focuses only on Traditional Chinese characters.