The menu item File > Session > Save Session (All Windows) saves the sizes and locations of all browser windows including all the tabs into a file.
The menu item File > Session > Save Window only saves the size and location of the current browser windows including all of its tabs into a file.
When loading such a file, iCab will open all the saved windows and tabs in their original location and size and in their saved state. In the Window settings you can configure if opening a "session" file will open the new windows and tabs in addition to the existing windows, or if the windows of the session file will completely replace the existing windows and tabs.