kmail: what are server-side sub, local sub?

I'm using kmail with dimap.  I see options for serverside subscription, and 
local subscription.  What do these mean?

I have my mail sorted on the server (lots of mail lists).  Some I'm not 
interested in anymore.  I could just delete them (through kmail).  But, I 
thought I could use unsubscribe in kmail so I wouldn't see them all, but 
they'd still be on the server if I wanted them.