It is for FTP, SMTP, POP3, IMAP at least.
This option has no effect if PORT, EPRT or EPSV is used instead of PASV.
.IP CURLOPT_USE_SSL
Pass a long using one of the values from below, to make libcurl use your
-desired level of SSL for the FTP transfer. (Added in 7.11.0)
+desired level of SSL for the transfer. (Added in 7.11.0)
+
+This is for enabling SSL/TLS when you use FTP, SMTP, POP3, IMAP erc.
(This option was known as CURLOPT_FTP_SSL up to 7.16.4, and the constants
were known as CURLFTPSSL_*)