The mobile app allows users to add new customers in the field, or to change the details of existing ones. It also allows them to upload these new or changed customer records to the web server.
As with orders, they hosting service allows uploaded customer records ti be downloaded as a CSV file, and uses change flags to acoid downloading records more than once.
The main difference is that mobile users can be expected to make legitimate changes to existing customer records. They cannot do this with orders, because orders cannot be modified once they have been sent.