taxonomy of prefixes

standalone (no more specifics, no less specifics)
root (have more specifics, are not more specific)
more specifics - subsets
subsets of other prefixes
express fine-grained policies
litter routing space?
getting free ride?

taxonomy by prevalence in tables:
globally visible: all BGP tables
highly visible: in most BGP tables
semi-global: in 1/2 of all BGP tables
local (a few tables)

highly visible ~= semi-global 
"bathtub curve" phenonenon (fuzzy dichotomy)