From Gnash Project Wiki
Every SWF can define a "symbol library", that is a collection of "characters" each with an associated "linkage name". The "linkage name" can then be used to reference the characters from ActionScript or from other movies.
Tipical use for the symbol library is 'attachMovie()' or 'registerClass()' AS code. Another use is IMPORT swf tag, specifying characters to be imported from another SWF by name.
Visibility of symbols from external movies
Imagine Main.swf loading Sub.swf.
- Symbols from Main.swf can always be attached to Sub.swf or its childs.
- Symbols from Sub.swf can be attached to Main.swf only if exported for runtime sharing (RSL)
This is new for me (strk) so no info on it yet, but it looks like it is something we should look at. Gnash doesn't support this.