The Doctor states that by the intergalactic law of the Shadow Proclamation seeding a level five planet is illegal (Partners in Crime).
Besides implying that Earth is level five, what else do we know about level five planets?
According to the Cubical 7 - Dr Who : Adventures in Time and Space RPG, planetary Technology levels are as follows.
That's the list, from what I can tell that is the Shadow Proclamation's view of tech levels. It's also in reference to the stages that the Earth has followed, assuming that every other life-sustaining planet went through the same periods.
As for the Atraxi being level 20 I do not think this is the case as the Timelords would be much higher in terms of tech levels, as they are claimed to be oldest and wisest race in the universe bar the Eternal's.
I have a feeling they are Tech level 7 because of the reason I have just given for the time lords. We can also see that they are not time faring. This is based upon the images the Atraxi show when the Doctor is questioning them. They show images of earth's history up until the Atraxi arrive. This leads me to believe that they have not yet developed time travel.
Also the Shadow Proclamation States that Level 5 and below are illegal because a higher level could alter the course of a planets technological evolution and in some cases if they are of a very low level the deities they worship.
To the best of my knowledge, planetary levels aren't really standardized throughout Doctor Who. It makes sense, if you think about it - the Doctor travels all over time and space, so he deals with an astronomical (literally) number of legal systems and classification scales.
Whatever intergalactic ruling body exists that the Adipose function within will have its own classification scale and its own rules about what makes up each level.
At minimum, we know (by the fact that Earth is a level 5 civilization) that level 5 has an existing native population of sapient creatures, has a technology level which permits rapid transport of materials on a continental scale, has lightspeed communication capabilities, has radio and other transmission capabilities, etc.
We don't know how many of those features come into play on this particular grading scale. We don't even know if a level 6 planet is more or less advanced than a level 5.
Unfortunately, like the exact details of the Shadow Proclamation and the caste system of the peoples of Raxacoricofallapatorius, the exact details of this system are unknown, and likely were never fleshed out beyond the needs of the plot.
It's simply there as a convenient way to demonstrate that the actions are illegal, the nanny knows it, and thus the villains will go to extreme lengths to keep their actions from becoming known.
According to the Doctor Who Wiki article on Planetary development, not much. The implication is that planets with relatively advanced sentient populations and organized civilizations are hallmarks of a Level 5 civilizations protected by Article 57 of the Shadow Proclamation.
We do know that Level 6 planets are ones capable of producing Sonic technology and that the galactic jailers, the Atraxi, are a level 20 civilization. Thus we could reason out that Level 5 is a relatively low level of technology, but one that has the potential for further growth or that it is a technology level that must be protected by galactic law, analogous to maybe the foreign treaties regarding how we interact with new countries and not interfering with their sovereignty.