DefaultTTL sets the TTL for all subsequent records following it in a domain that do not explicitly set one with TTL. If neither DefaultTTL or TTL exist for a record, the record will inherit the DNSControl global internal default of 300 seconds. See also DEFAULTS to override the internal defaults.
NS records are currently a special case, and do not inherit from DefaultTTL. See NAMESERVER_TTL to set a default TTL for all NS records.