This option specifies the SSL or TLS version that will be used for https connections.
The most compatible version is sslv23, will negotiate down from TLS 1.2 (most secure) to TLS 1.1 to TLS 1.0 to SSL 3.0 and finally to SSL 2.0 (least secure). All the other versions are strict and require specific support. For example, if you configure the ACS to use tlsv1_2, the CWMP client must support TLS 1.2 or the connection will fail. The ACS will not negotiate down to a lesser version.
Section 3.3 of TR-069 Amendment 5 states that SSL 3.0 and TLS 1.0 SHOULD NOT be used. Likewise, this section also states that the CPE should use TLS 1.2.