>>So, the format you provided is mainly a common format use in such situations. But, as you said, in order to our format to work, I just need to add the Body tag.
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>It's not that you need to add the Body tag: the Body element is mandatory in SOAP messages. This should be relevant, of course, only if you (/your client) are somehow interested in following a specific standard. If their particular specs define a different XML messaging mechanism, you'll have to comply, won't you? But I think it would be better for them - and easier for you - to follow the SOAP standard.

Well, so far, it hasn't be the case. So, I guess the specs I received will remain as is. I'll just proceed by making them a special error XML string when an error would happen, if it happen, and for any other validation messages they want to receive.

I mostly wanted to get some more technical info on this in overall and you have provided them. Thanks