STX refers to the Streaming Transformations for XML language and should be used on questions related to the usage of the language.
Streaming Transformations for XML (STX) is a one-pass transformation language for XML documents. STX is intended as a high-speed, low memory consumption alternative to XSLT, using the W3C XQuery 1.0 and XPath 2.0 Data Model. Since STX does not require the construction of an in-memory tree, it is suitable for use in resource constrained scenarios.
It resembles XSLT syntactically, however being a streaming language it does not need to load the full DOM into memory the way XSLT does.
This language is very much a niche language, and the future of it is somewhat uncertain given that XSLT 2.1 should have some streaming functionality implemented and that there is really only one implementation of it - Joost http://joost.sourceforge.net/