Garden-worlds are planets that can sustain standard human ( Nitrogen/Oxygen breathers) without extensive technological assistance (unprotected).
The Union Colonist and Settlement Bureau has developed a Garden world Classification chart, that determines the level of compatibility.
There are 7 Base Types and a grades from A to F. A= Perfect, F= Fail
Earth for example is a Type 1B Garden-world. While it is perfect in many regards, it does have tectonic activities, lifeforms and climate zones that can be dangerous to unprotected human life.
Only a few planets like Azure or Para-Para received the rare Type 1A classification.
Most planets fall under the Type 3D type.
Ulta, Nilfeheim and New Sweden are examples of Water-worlds (Ulta 4B, Nilfeheim 4D and New Sweden 4A)
Note that this Classification chart is not used by the Fleet or the Science Corps, but has found its way in daily conversations and of course is used by prospective Settlers and Explorer - Survey crews.
Also note that Non Garden worlds are often also settled but are not reflected on this chart.
They are classified as Rock or Rock Core planets and hjave their own classification
There are also Gas giants, Ice giants and Failed suns