SpamAssassin client program
This option determines if spam filtering is done using the standalone spamassassin program, or the spamc lightweight client. In the latter case, your system must be running the spamd server, or no spam classification will be done.

Using spamc is typically faster and consumes less CPU. However, it will take away the ability to have a different SpamAssassin configuration for each virtual server, as the spamd server can only read a single set of configuration files. Using spamc/spamd also consumes more memory.