a tier-1 network does not get transit prefixes from any other network and peers with, among others, other tier-1 networks.
a tier-2 gets transit of some form from another network, usually but not necessarily a tier-1, and may peer with other networks.
this does not please everyone, especially folk who buy transit and don't like discussing it. and there are kinky corners Even this is debatable (& I know you know this Randy).
in this forum, everything is debatable. some portion of the debate makes sense. ymmv.
Firstly, peering isn't binary. Is peering vs transit a distinction based on routes taken / accepted & readvertised, or on cost? Does "paid for peering" count as peering or transit? If you pay by volume? If you pay for "more than your fair share" of the interconnect pipes? (if the latter, I am guessing there are actually no Tier 1s as everyone reckons they pay for more than their fair share...).
pay? did i say pay? i discussed announcement and receipt of prefixes. this was not an accident. it is measurable.
Secondly, it doesn't cover scenarios that have have happened in the past. For instance, the route swap. EG Imagine networks X1, X2, X3, X4 are "Tier 1" as Randy describes them. Network Y peers with all the above except X1. Network Z peers with all the above except X2. Y & Z peer. To avoid Y or Z needing to take transit, Y sends Z X2's routes (and sends Z's routes to X2 routes marked "no export" to X2's peers), and Z sends Y X1's routes (and sends Y's routes to X1 marked "no export" to X1's peers). Perhaps they do this for free. Perhaps they charge eachother for it and settle up at the end of each month. Perhaps it's one company that's just bought another.
seems to me that, if you look at the prefixes, it's pretty clear. randy