When representing empty categories in tree structures, linguists use a null symbol (∅) to depict the idea that there is a mental category at the level being represented, even if the word(s) are being left out of overt speech.
In linguistic representations of empty categories in tree structures, a null symbol (∅) is employed to signify that there is an implicit mental category at the current level of representation, which may or may not be accompanied by explicit word usage in spoken language.