The package[1] is a binding to the Expat XML parser[2] with no high level wrappers, just plain C API.
The initial release covers all of expat 2.2.0 and you need to link against the appropriate library yourself.
The compile-time versions XML_UNICODE, XML_UNICODE_WCHAR_T, and XML_LARGE_SIZE correspond to the C defines by the same name; use them to match the binding to how expat has been compiled.

Rationale as to why this package: I needed a fast, (time) tested XML stream parser to use for XMPP parsing and so far I found no current D package for that.

[1] https://code.dlang.org/packages/expat-d
[2] http://www.libexpat.org/