What does "sa_update" function do in options.conf and same for spamassassin_cron

Hi All,
I am trying to find out what exactly the "sa_update" function does in options.conf and how to configure it.

I suppose I can set it to yes but I caot find a description in the custombuild help.

Do I set it to yes and run:

./build spamassassin_cron

Or do I rebuild spamassasin, and yes what does it do and what does the above cron option do?

thanks Joris