BitcoinQT (or bitcoind) is the most used full node implementation. As such it is considered a reference for other implementations. If an alternative implementation is not compatible with BitcoinQT it may be forked. That is, it will not see the main chain as the rest of the network running BitcoinQT.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc