wsrp:maximize vs. wsrp:solo

Can someone refresh my memory on our intent in distinguishing between a 
state where we say that the portlet has more display space then normal 
[wsrp:maximize] and one where we say this is the only portlet being 
aggregated [wsrp:solo]?  I.e. for what distinguishing purpose did we add 
wsrp:solo?
    -Mike-