ESCI stands for e-Supply Chain Interface - it is a facility for automating the orders/deliveries/invoices dialog between the practice and its suppliers.

You don't have to use it, and it requires setting up at both the OpenVPMS and supplier ends. Both Lyppards and Provet in Australia have implemented their parts.

To quote from the documentation:

The OpenVPMS ESCI (e-Supply Chain Interface) is a standards-based API to enable OpenVPMS to electronically place orders on suppliers and to receive order responses and invoices from suppliers . It is based on the exchange of Universal Business Language (UBL) 2.0 documents via web services.

ESCI defines a number of web services which are implemented by the supplier, namely:

• Order – enables clients to submit orders to the supplier
• Inbox – enables clients to receive documents posted by the supplier
• Registry - provides a simple lookup mechanism to resolve the other supplier web services

It works as follows:

• OpenVPMS creates UBL Order documents and submits these to the supplier via the supplier’s Order web service.
• Orders are processed asynchronously by the supplier.
• A UBL OrderResponseSimple document is placed in practice’s Inbox, indicating success or failure.
• When an order has been shipped, the supplier invoices the practice by placing an UBL Invoice document in its Inbox.
• OpenVPMS checks the Inbox web service regularly to process any incoming Order Response and/or Invoice documents.

If you are going to set up ESCI, then this link provides useful information.