Worldwide there are currently 15 different types of domestic plugs & sockets in use.
What kind of domestic plug & sockets is used, is defined in the national standards. Most of those standards are listed in the IEC TR 60083.
To distinguish the difference between the domestic plugs & sockets, each of them is added by IEC with a letter.
These letters are completely arbitrary and have therefore no meaning.
Here is an overview of the domestic plugs & sockets which Bals Elektrotechnik marketed.
More information, as well as a full list of the worldwide used domestic plugs & sockets you can find here.