This took a bit for me to figure out. The difference between sigils and words is their primary purpose. Words, letters, ideograms and other such symbols are used to communicate ideas between the writer and the reader. Therefore they must be federated between the writer and reader in such a way that both understand what the definitions for these symbols mean before any ideas can be communicated between them. Sigils on the other hand are primarily used as a neumonic device for visualization of one casting or weaving a spell. They are used for communicating with the divine or universe at large and therefore do not require to be federated. More to the point they are a tool to commuicate one’s own subjective truth and perception and therefore it’s best to NOT federate sigils but rather create one’s own.
Another way to look at is while language is used to communicate objective ideas between people and requires to be federated and objectively agreed on between it’s users, sigils are the complete opposite that are used to determine subjective truth in it’s purity and therefore federating has limited value. Words are to logic as sigils are to art. And yet much as in math, science, art or writing, there is much cross over between the two. After all a symbol can mean the same thing to many people and help focus the mind just as well.