The WS-I Basic Profile (official abbreviation is BP), a specification from the Web Services Interoperability industry consortium (WS-I), provides interoperability guidance for core Web Services specifications such as SOAP, WSDL, and UDDI.
To understand the importance of WSI-BP, note that it defines a much narrower set of valid services than the full WSDL or SOAP schema.
Compare the WSDL 1.1 specification to the subset permitted in WSI-BP.
Also note that WSI-BP generally narrows the SOAP specification.
There is a notable exception where WSI expands on the SOAP standard, and that is in adding xml:lang attribute on fault elements.