Field help (AYILNKADS) 

Code of the XTEND pool if it is different from the default pool entered at the level of parameter 'Web site'.

Using a specific XTEND pool for a given interface makes it possible to distribute the load between different connection pools when few Web service licences are available.

For instance:

The default pool can be used for calling interactive Web services having short response times.
However a specific pool can be used for calling Web services triggering longer processings (such as passing of an order) so that the other Users are not blocked.
This pool can possibly be connected to a dedicated processing server.