On a lookup, both a Name and a SKU node must be passed.
On an update, if you want to set a field to NULL or "empty string" use an empty element syntax (e.g. <Summary></Summary>).
On an update, if you want to leave the existing value on that field, omit the field from the Xml input document.
If an Update, Nuke, or Delete is done, the request should have an ID or GUID for the best processing. If none is provided, a lookup will be done based on the combination of product name and SKU, but this will slow down processing, especially on bulk updates.
Incremental updates are supported
You can assign your own GUID on product Add. The GUID and new ID will be returned to you for association within your calling system.
UseAppConfigs on the Image node tells WSI to use the image resize AppConfigs for any images you provide.
If the EnsureDefaultVariant attribute is set to true, WSI will pick one of the product's variants and flag it as the default.
Warnings:
Remember that between product data, variants, kits, inventory, entity mappings, etc alotof tables are involved in product data. Be very sure that you understand the DB structure and how products are put together before doing any bulk product updates.