Both allow for a degree of modularity in their languages, including, for example, splitting the schema into multiple files.And both of them are, or can be, defined in RELAX NG does not have any analog to PSVI.
A schema is analogous to a grammar for a language; a schema defines what the vocabulary for the language may be and what a valid "sentence" is.
There are historic and current XML schema languages: The main ones (see also the ISO 19757's endorsed languages) are described below.
For WXS validators that will follow a URI to an arbitrary online location, there is the potential for reading something malicious from the other side of the stream.
W3C XML Schema does not implement most of the DTD ability to provide data elements to a document.
W3C XML Schema has a rich "simple type" system built in (xs:number, xs:date, etc., plus derivation of custom types), while RELAX NG has an extremely simplistic one because it's meant to use type libraries developed independently of RELAX NG, rather than grow its own. In practice it's common for a RELAX NG schema to use the predefined "simple types" and "restrictions" (pattern, max Length, etc.) of W3C XML Schema.
In W3C XML Schema a specific number or range of repetitions of patterns can be expressed whereas it's practically not possible to specify at all in RELAX NG ().
Validation of an instance document against a schema can be regarded as a conceptually separate operation from XML parsing.
In practice, however, many schema validators are integrated with an XML parser.
Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG.