To be effective does a symbol need to acquire a conventional or accepted meaning?

I would think, yes. Symbols are to visual experice what acronyms are to language. They are a sort of shorthand communication. If I say "FBI" chances are very good you know what I mean. But if I say "KPX" chances are equally good that no meaning is conveyed. Why? Because there is no accepted meaning for KPX. We have no shared experience of KPX.