How to use SyncMapping service

I could use some advice on the syncmapping table.  I am doing an product extension and trying to figure out what values should go in the "SourceKey" and "ContextName" fields.

I think the ContextName will be the fieldname I am trying to save, but not positive.

Thanks in advanced.

For a better illustration... let's assume you want to import products from an ERP system called MyERP:

• ContextName: MyERP (any name which identifies the external system, plugin, app or whatever)
• EntityName: Product (makes sense if you deal with multiple entity types from the same source)
• SourceKey: [ProductIDFromMyERP] (the external system's Id/Key for this product entity)
• EntityId: [ProductIDInSmartStore] (the ID of this entity in the Product table) 

Thank you for the reply but I'm still a bit confused about one part.  

There is an index:

• ContextName: MyPluginName
• EntityName: Product
• SourceKey: null (I am just saving extraneous data about the product and there is no link)
• EntityID: ProductID

This would cause duplicates because there all products would have the same value for ContextName, EntityName, SourceKey.  It seems like I don't have any choice but to make SouceKey = ProductID.

And in my case I need to save multiple extra fields for one product so either I have to change EntityName to be the fieldname or I would get duplicates on just one product.  I could do SourceKey = ProductID + FieldName but then I would have to parse it out. 

Thoughts?

Thanks

Just another example:

var syncMapping = _syncMappingService.GetSyncMappingByEntity(customer.Id, "Customer", "MyCompany.MyPluginName");
if (syncMapping != null)
{
	syncMapping.CustomString = JsonConvert.SerializeObject(my_model_instance);
	_syncMappingService.UpdateSyncMapping(syncMapping);
}
else
{
	syncMapping = new SyncMapping
	{
		EntityId = customer.Id,
		EntityName = "Customer",
		ContextName = "MyCompany.MyPluginName",
		SourceKey = yourExternalId,	//optional
		CustomString = JsonConvert.SerializeObject(my_model_instance)
	};
	_syncMappingService.InsertSyncMapping(syncMapping);
}