s_webentryhost — Name of the host that receives the first HTTP request from the desktop client
s_external_url — URL that third party tools use to connect to the E-Business Suite System. This is used only by the Oracle Web Services product.
2.1.4: Configuration Changes on the Load Balancer
The Hardware Load Balancer must be configured for session persistence: various modules shipped with Oracle E-Business Suite need to maintain session state. If this property is not set, the loss of transaction state may result in users experiencing unpredictable errors.
Oracle supports session persistence technology that is transparent to the Oracle E-Business Suite; that is, session persistence approaches which do not require additional programmatic changes to the underlying E-Business Suite modules. For successful integration of your load balancer with Oracle E-Business Suite application tiers, refer to your load balancer’s documentation for details on how to configure the load balancing pool, virtual host configuration, and session persistence parameters. Networking hardware vendors offer products with a range of technical solutions for session persistence, but it is beyond the scope of this document to make comparative assessments of the various approaches.
Oracle E-Business Suite Installation Guide: Using Rapid Install
Release 12.1 (12.1.1)
Part Number E12842-04
The value of s_webentryhost is if you are using a load balancer to distribute processing across all the Applications tier nodes.
There is one good document you could refer as well
Implementing Load Balancing On Oracle E-Business Suite – Documentation For Specific Load Balancer Hardware ( Doc ID 727171.1 )
Oracle E-Business Suite 12 (BIG-IP LTM, WebAccelerator, FirePass)