If you are not familiar with how sessions are configured in WebSphere, check out our getting started page.
By default, sessions are managed at the application server level. For example, let's say you want to change the maximum allowed sessions. At the application server level (Server > Server Types > Websphere applications servers > select an application server > Session management), you will be able to modify the maximum allowed sessions.
However, at the application level (Application > Application Types > Websphere enterprise applications > select an application > Session management), the options are greyed out, as these options are controlled at the application server level.
The reason the options are greyed out at the application level is because override session management is not enabled by default. When you would like to configure unique sessions management settings at the application level, you will checkmark override session management and then the options at the application level will no longer be greyed out.