This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
X stands for eXtension. Any "X" parameter is vendor-specific, and unlike the other options isn't guaranteed to be available on all JVM implementations - for example, IBM's jikes or the open-source IcedTea VMs.
An IDE is no substitute for an Intelligent Developer.