The ID represents a unique object in LAC and is present in most of the API functions. When you are adding a new object, you should leave the ID field unset or set to zero; for example, in AddCustomer:
<customer xsi:type="ns:Soap_Customer">
<!--You may enter the following 12 items in any order-->
<id xsi:type="xsd:int"></id>
<name xsi:type="xsd:string">Customer Name</name>
...
</customer>
Updates are made based on this ID, so when modifying an existing object, it is important to set the ID; for example, in UpdateCustomer:
<customer xsi:type="ns:Soap_Customer">
<!--You may enter the following 12 items in any order-->
<id xsi:type="xsd:int">1</id>
<name xsi:type="xsd:string">Customer Name</name>
...
</customer>