SASL stands for “Simple Authentication and Security Layer”. SASL itself is nothing more than a list of requirements for Authentication (SASL) Mechanisms and protocols to be SASL-compatible as described in RFC 4422. IMAP, POP3, SMTP, and ManageSieve protocols all have support for SASL.
Many people confuse SASL with one specific SASL implementation: the Cyrus SASL library. Dovecot has its own SASL implementation which could (one day) be separated from Dovecot itself to “compete” against Cyrus SASL library as an alternative implementation.
Dovecot can be used as the SASL server for several external SMTP/Submission servers. See SMTP AUTH.