If you try to send XML as below, only the first field will be correctly interpreted by the XML parser used in SpiraTeam
<RemoteTestCase xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/Inflectra.SpiraTest.Web.Services.v5_0.DataObjects">
<TestCaseStatusId>6</TestCaseStatusId>
<Description>Description of the new test case</Description>
<OwnerId>45</OwnerId>
<Name>The best test case ever</Name>
</RemoteTestCase>
To make sure that all 4 fields will be received correctly by SpiraTeam, so that the API call will be successful, you need to send the XML with all tags in alphabetical order - as below.
<RemoteTestCase xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/Inflectra.SpiraTest.Web.Services.v5_0.DataObjects">
<Description>Description of the new test case</Description>
<Name>The best test case ever</Name>
<OwnerId>45</OwnerId>
<TestCaseStatusId>6</TestCaseStatusId>
</RemoteTestCase>
If using JSON, the field order is not important.