The container settings dialog must be used to customize all settings which are not global and therefore not available in the Miranda options. Since TabSRMM supports multiple containers, these containers can have different settings for appearance and behavior. For example, you could define 2 containers, one for normal IM chats and one for group chat sessions (e.g. IRC, Jabber rooms) and configure them with a different window layout and different options.
A container options dialog can only exist when its owning container window (a message window with one or more tabs) is open. When you close the message window, the options dialog will automatically close and all changes you have done to the options are lost.
By default and to simplify the settings, TabSRMM uses shared settings for all containers, so when you change the options for the "Default" container, all other containers will inherit them. This is fine in most cases, but there are exceptions. Consider you have a dedicated container for all group chat sessions while all normal (IM) chats go into the default container. You may want to configure the group chat container in a different way and that's where private container options can be useful. At the bottom of the container options dialog, you can find a check box which is usually unchecked. Once checked, the container will use its own set of settings.
When a container uses private settings and you change the height of the info panel or the splitter position, it will only affect this specific container. Consequently, when you change one of these things in a container that uses global/shared options, all windows will be affected.
Because these settings are bound to a specific container, a message window must be open. It is not possible to invoke container options from outside the message window.