@Graham Miln ha risposto alla mia domanda, ma ho trovato questa spiegazione se qualcuno volesse informazioni aggiuntive:
There is not a practical difference between both "string" and "text" terms, concerning AppleScript.
However, there are some "theoretical" differences:
The main difference between "text" and "string" belongs to application developers, since they can define their own "type" ("TEXT" for string, "ctxt" for text) when they pass texts to AppleScript (eg, they can pass style information, as the clipboard, only Mac-ASCII characters, as Tex-Edit Plus, or even Unicode text -another kind of text-, as the Finder).
Fonte:
MacScripter.net / Qual è la differenza tra "stringa" e "testo"?