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:
It works as follows:
If you are going to set up ESCI, then this link provides useful information.