How to use the tag Key?

BlackBird

Hi all,

When publishing a Page based on t 37 (Sales Line) as a webservice there is a tag <Key>. See the linein red:

soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/&quot; xmlns:ord="urn:microsoft-dynamics-schemas/page/order">
<soapenv:Header/>
<soapenv:Body>
<ord:UpdateMultiple>
<ord:Order_List>
<!--1 or more repetitions:-->
<ord:Order>
<!--Optional:-->
<ord:Key>?</ord:Key>

how should one use this field? Is it used to select a key of table 37? What is the syntax?
I have tried several values but i keep getting the error: Key "" is not wel formed!
Best regards and thanx for your help