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If you send in undefined elements, depending on the WarningLevel in your request, you may see elements ignored or get warnings on your requests.
Assume you have WarningLevel set to High and you send in an AddItem request like this :
<Description>An eBay original</Description>
You will receive a warning like this :
<ShortMessage>Unrecognized element <Item.Subtitle> in request message.</ShortMessage>
<LongMessage>The element <Item.Subtitle> was found in the input request message. This is not a declared element in the schema and will be ignored.</LongMessage>
As the warning indicates, the <Subtitle> is not recognized (should be SubTitle with capital 'T') and will be ignored.
If you do not have the WarningLevel set to High, your Subtitle (sic) will be ignored and you won't see it in your listing.