These content scanning plugins provide capabilities for performing various kinds of additional content scans before DansGuardian releases a file to a user. Often all that's needed is to enable/disable (uncomment/comment) individual ones, which can be done via the "plugins" tab in the main config editing task window. They are used mainly for anti-virus scanning.
Do not enable both clamav and clamdscan at the same time. They provide approximately the same functionality, so both would just use up a lot of additional CPU resource without providing better virus protection. The clamdscan method is more flexible and is preferred in all cases. The clamav method --which can also complicate building-- is present mainly for backward compatibility. (The --enable-clamav build option is not necessary to use clamdscan; only the --enable-clamd build option is necessary.)
If you wish to implement a scanner that's not included in the list of pre-supplied options (for example some other kind of anti-virus, or some other type of scan entirely such as one that tries to distinguish pornographic pictures), use the commandline choice.
Each plugin has a config of its own. In many cases these configs don't need to be changed, If one of the configs does need to be changed, you can access the config file for editing through the plugin config editing tool.
Some of the options may say #!! Not compiled !!. If so, no config combination can ever enable any of these options on your system. To make such options available, you must get both a DansGuardian executable that includes support for the option and config files that match it. One way to do this is to rebuild DansGuardian from source. Rebuilding includes running ./configure, compiling, linking, and installing; it should not be undertaken lightly.