In inline mode, traffic passes into one of the appliance's Ethernet ports and out of the other. When two sites with inline appliances communicate, every TCP connection passing between them is accelerated. All other traffic is passed through transparently, as if the appliance were not there.
For example; when you have an SD-WAN appliance deployed in Inline-Mode with only 4 VLANs on interface group. If there are 6 VLANs going through the traffic, only trafic that matches the 4 VLANs configured is recognized by the appliance.
Any TCP-based traffic passing through both units is accelerated. No address translation, proxying, or per-site setup is required. Inline mode is auto-detecting and auto-configuring.
Configuration is minimized with inline mode, because your WAN router need not be aware of the appliance's existence.
Depending on your configuration, inline mode's link-down propagation can affect management access to the appliance if a link goes down.
Inline mode is most effective when applied to all traffic flowing into and out of a site, but it can be used for only some of the site's traffic.