A depot is a list of products and the number of each products that are available in the depot, eg. number of bottles of soap. Each company can have a depot.
You can create depot order lines from the CRM module, and these lines can then be exported from the Invoices module. The lines can either be exported as a CSV file or they can be mailed as an Excel file; this is configured in the DEPOT EXPORT menu in the Invoices module (the permission for it is actually setup in the Users module).