Use of '@' '.' and 'public operator/keywords in Oracle database?
This diagram is from the Oracle Docs Syntax for Schema Objects and Parts in SQL Statements page:  



It shows the general use of the period character (.) for qualification and the @ symbol for referencing dblinks.

The @ symbol is also used when creating dblinks.

Given that the public keyword is also used for creating them, I think it is likely that your external function is doing something with dblinks!