The SymbolClass tag is used to instantiate objects from action script version 3 definitions (see DoABCDefine.) You can instantiate each object only once with this technique.
The f_symbol_id references an ActionScript version 3 object (DoABC) and the f_symbol_name references the class to instantiate.
When f_symbol_id is set to zero, this tag becomes a special case and uses the f_symbol_name as the name of the top level class (root? TBD.)