It's a semi-unofficial tier that it still referenced by the Smogon statistics. Basically it's the bottom rung, it's the NU of NU and it's name is a pun. It doesnt have official status for battling servers on smogon but they accept it as a tier that exists so I will edit the tier list thread with this as it's useful information if you want to make the big NU tier smaller to have even lower down battles.
Using the same methods as determining other tiers, all the most used NU mons in NU are banned from PU and everything under that is allowed.
Here are the banned NU mons going by the recent March Shifts:
Charizard Wartortle Raichu Vileplume Kadabra Golem Haunter Exeggutor Marowak Weezing Tangela Jynx Pinsir Tauros Flareon Quagsire Misdreavus Miltank Ludicolo Swellow Gardevoir Ninjask Camerupt Torkoal Cacturne Altaria Armaldo Absol Gorebyss Regirock Torterra Luxray Bastiodon Floatzel Drifblim Skuntank Lickilicky Magmortar Leafeon Probopass Rotom-S Serperior Emboar Samurott Musharna Gurdurr Sawk Garbodor Cinccino Duosion Sawsbuck Amoonguss Alomomola Eelektross Cryogonal Braviary
so that leaves all these mons to be used in PU:
Ampharos
Arbok
Ariados
Articuno
Audino
Banette
Basculin
Beartic
Beautifly
Beedrill
Beheeyem
Bellossom
Bibarel
Butterfree
Carnivine
Carracosta
Castform
Chatot
Cherrim
Chimecho
Chinchou
Clamperl
Combusken
Cottonee
Cradily
Delcatty
Delibird
Dewgong
Ditto
Dodrio
Dragonair
Dunsparce
Dustox
Dwebble
Electabuzz
Electrode
Emolga
Exploud
Farfetch'd
Fearow
Fraxure
Furret
Gabite
Gigalith
Girafarig
Glaceon
Glalie
Golbat
Golduck
Gothitelle
Granbull
Grumpig
Heatmor
Huntail
Hypno
Illumise
Jumpluff
Kangaskhan
Kecleon
Kingler
Kricketune
Lairon
Lampent
Lapras
Leavanny
Ledian
Liepard
Linoone
Lopunny
Lumineon
Lunatone
Luvdisc
Magcargo
Mantine
Maractus
Masquerain
Mawile
Meganium
Metang
Mightyena
Minun
Monferno
Mothim
Mr. Mime
Muk
Murkrow
Noctowl
Octillery
Omanyte
Pachirisu
Parasect
Pelipper
Persian
Phione
Pidgeot
Pikachu
Piloswine
Plusle
Purugly
Rampardos
Rapidash
Raticate
Regice
Regigigas
Relicanth
Rotom-F
Sandshrew
Scraggy
Seadra
Seaking
Seismitoad
Seviper
Shedinja
Shelgon
Shiftry
Shuckle
Simipour
Simisage
Simisear
Skorupi
Slaking
Sneasel
Solrock
Spinda
Stantler
Stunfisk
Sudowoodo
Sunflora
Swalot
Swanna
Swoobat
Throh
Togetic
Trapinch
Tropius
Unfezant
Unown
Ursaring
Vanilluxe
Vespiquen
Victreebel
Vigoroth
Volbeat
Wailord
Walrein
Watchog
Whirlipede
Whiscash
Wigglytuff
Wormadam
Wormadam-G
Wormadam-S
Zangoose
Zebstrika
Zweilous
so it will be interesting to see how PU shapes up as a tier and if it gets enough players trying to do something interesting. So this thread is to discuss it.
here are march's statistics if you want the stats:
http://www.smogon.com/forums/showthread.php?t=3464495