simply, it means when an `s type` planet is explored, it is possible it can be a lot larger in mass than the avaerage of that planet type. for example, i once explored a desert planet which was around 1300-1400 in land mass. this planet usually caps out at a couple of hundred land.
to be pre-emptive, an X type planet (which is explored on occasion in the same way as an S type) cannot be plundered unless you are a collective or marauder (i believe).
a u.fertile is exactly what it sounds like. useful for agriculture empires and greatly increases the amount of food produced per land.
a u.large is a planet which is explored and can be up to around 10,000 in land mass.
a u.eden populates very quickly.
a u.rich is bollocks.
a u.spazal is used for digging for artifacts - the lower land mass the better (usually 2 for digging but ocassionally 1)
any planet beginning with `U.` cannot be clustered.
hope this gave you a little insight :)