At a very very basic level, VDSL Band Status, U0, U1, U2, D1, D2 & D3 are all different upstream and downstream frequency bands, each has a different attenuation because of the different frequencies.
(Attenuation – ‘the reduction of the force, effect, or value of something’.).
In essence, the data is carried down or up the wire in three separate chunks (frequencies)
On longer lines then the higher U and D values are dropped as they are run at higher frequencies and higher frequencies do not travel as far along a line as lower frequencies, (bit like 5GHz wifi as compared to 2.4GHz wifi ) which then results in a lower throughput, in UK we generally have 3 up and 3 down.
Hope that helps give the basics?
In a completely sane world, madness is the only freedom (J.G.Ballard).