compatibility option is only available when avast detects that u have another antivirus pre-installed on ur system (if it is a clean system, no such option is provided, with other av preinstalled, the option is located below the 'express install' option).
edit: tried the compatibility mode on a VM, configuration is avg as primary av and avast as secondary av. the real-time shield installed in avast is the script shield.
u could also use avast as an on-demand scanner/boot-time scanner.
under additional protection, webrep and remote assistance is available. autosandbox is shown there too, however, i doubt it is available as it is dependent on real-time file system shield, which is not present.