Webservices and Groups

SUGGESTED

In a webservice you have to select a group for a parameter/value. What is the purpose of that? 

Why do i get the following message,

[err] Field ZBPCEMP not expected into group GRP0.

when  in my webservice definition the field is in Group0! Also in my Body of the POST-Request i put it in Group0

<inputXml xsi:type="xsd:string"><![CDATA[<?xml version="1.0" encoding="iso-8859-1"?>
<PARAM>
<GRP ID="GRP0">
<FLD NAME="

...
<FLD NAME="ZBPCEMP">11147</FLD>

....


</PARAM>]]>
</inputXml>

I found no restriction in the subprogram, nor in the object definition. So what purpose do these groups have? And where is this validation message from? 

Thanks in advance for every help.