Features
There are a number of features which distinguish XMPP.
Choice of clients and platforms
XMPP is not restricted to Smartphones.
Multiple Accounts for different uses
Some chat systems are ‘flat’ – using a phone number as identifier. This can increase the risk of accidentally adding an unintended person to a chat.
XMPP Clients will generally connect to more than one account at a time, thus someone who is a teacher, a voluntary fireman and a scout leader could have three accounts, one for each role, but all can be enabled simultaneously.
Note that there are mobile phone providers who do offer an XMPP service, for those who might want a chat address not clearly connected with some organisation.