It's neither a mysqldump feature nor a directive code or name. It's good old MySQL conditional comments:
If you add a version number after the “!” character, the syntax within the comment is executed only if the MySQL version is greater than or equal to the specified version number. The TEMPORARY keyword in the following comment is executed only by servers from MySQL 3.23.02 or higher:
CREATE /*!32302 TEMPORARY */ TABLE t (a INT);
The comment syntax just described applies to how the mysqld server parses SQL statements. The mysql client program also performs some parsing of statements before sending them to the server. (It does this to determine statement boundaries within a multiple-statement input line.)
Comments in this format, /*!12345 ... */, are not stored on the server. If this format is used to comment stored routines, the comments will not be retained on the server.
Thus 50500
stands for MySQL 5.5.0 or greater. It's a way to ensure that dumps can get restored in older servers.