A binding is the mechanism by which a user can connect to the website. Perceptive Enterprise Search supports two forms of bindings, IP and IIS. A website can have multiple bindings, which may be required when hosting a web server inside a DMZ, or due to other networking issues.
It is also possible to have both an IP binding AND an IIS binding to the same website, however this may result in odd security behavior.
When a website is IP bound, Perceptive Enterprise Search controls the security. This includes challenging the user for their credentials and validating their login.
When a website is IIS bound, its security mode is set to "Hosted". This instructs Perceptive Enterprise Search that the hosting web server (IIS) is taking care of security, and no further security actions are required by Perceptive Enterprise Search.
These two security modes are mutually exclusive, you must pick one or the other. For this reason, it is not recommended to use mixed bindings.