SASL is an authentication protocol, which improves the way e.g. NickServ login is performed, allowing connections to be established more quickly.
What is the advantage of using SASL over logging in with a client SSL certificate?
What is the advantage of using SASL over logging in with a client SSL certificate?
Also, try explaining client certificates to non-technical people, who probably are among the target audience of IRCv3 initiative.