you basically can put into the settings of a security anything you like. You even can rename your security completely.
Internally you can address the security thru the ISIN, e.g. in case you want to scan bank documents automatically. And the ISIN in fact is unique to the security.
For the download of (historical) quotes, however, PP takes the symbol. With the symbol you can direct PP to the stock exchange you want to get the quotes from, and even the currency in some cases.
For SPIE you can get the quotes from Paris with the symbol SPIE.PA, for the same security you can get the quotes from Berlin with 4SP.BE, or from the NASDAQ in USD (!Watch out for the currency settings for the security! The numbers PP obtains from stock exchanges are currencyless, i.e. just numbers and get their currency through your settings.) by the symbol SPIWF.
I hope this helps