How does portupgrade -ar work?

[Lars Stokholm]

First of all, is it safe to just use 'portupgrade -a' or should I use 
'portupgrade -ar'.

Secondly, I've seen people use 'portupgrade -arR', but isn't that 
completely unnessesary?

And finally, how does 'portupgrade -ar' work? Let's say two ports were 
updated, and an upgrade was required: libiconv and libslang. Would 
portupgrade be intelligent enough to upgrade both before upgrading slrn 
(which depend on these)? Or would portupgrade first upgrade libiconv and 
slrn, and then libslang and slrn again?

Are there (other?) problems with using 'portupgrade -ar'?

-- 
Regards
Lars Stokholm