Speaking in a very general sense, second level domains belong to the organization who registered them, and are therefore their responsibility to resolve. Using IBM for an example, IBM is responsible for ibm.com. The IBM part of ibm.com is the 'second level'.
For country code domains, such as Taiwan's .tw, the top level is extended to include .com.tw, .net.tw, .org.tw, etc. Companies such as Abit (computer component manufacturers) would be abit.com.tw. Because the root registry in that region controls both the country code domain and the organization level domain, the owner is still the second level domain. Thus 'abit.com.tw' is still considered a second level domain.
In strictest resolution sense, abit.com.tw is actually a third level domain. Just exactly what you call such a domain is a bit esoteric and depends on the domain and who you are discussing it with. Generally, it is accepted that the term 'second level domains' are managed and owned by the organizations that purchased the right to register that particular domain name.