There's a reference to a level nine forcefield being used in Voy: Killing Game. There's no indication why a level ten forcefield isn't being used.
JANEWAY: Damn, there's a level nine forcefield around the surgical console. We'll have to go in and shut it down.
JANEWAY: I'll explain later. Set the detonator for five minutes.
Similarly, a level eight forcefield is used in DS9: Rejoined, again, there's no obvious reason why they wouldn't use a more powerful field.
EDDINGTON: There's no way to get to her.
DAX: How long will a level eight forcefield last in here?
EDDINGTON: Not long with that plasma fire. Ten, twenty seconds at most.
Level seven fields appear (twice) in Voy: Demon, both times used to contain a poisonous atmosphere.
JANEWAY: Well then, you're going to have to help me figure it out. Mister Tuvok, I'm on my way to transporter room one. Erect a level seven forcefield around the platform. Fill it with atmosphere from the planet and then beam Mister Kim, or rather his duplicate, aboard.
A level five forcefield is used to protect a potentially explosive (albeit small) container in TNG: Realm of Fear
LAFORGE: So they were collecting samples from the plasma streamer. They had the proper container. How could it have exploded?
DATA: Perhaps we should attempt to recreate their experiment to see what happens when matter is beamed aboard under similar
LAFORGE: That's a good idea. We'll prepare a new container. Reg, I don't want to take any chances here. Start setting up a level five
containment field here in Engineering, okay?
and again in DS9: The Alternate to contain a growing lifeform
O'BRIEN: Whatever it is, it keeps changing. Somehow it goes through a metamorphosis every time it reproduces, and it reproduces
like crazy. Look at this. It's multiplied so much I had to change
containers. I'll put it in a level five security field overnight. I
wish Dax were here to take a look at it. How's she doing?
And again in VOY: The Raven and VOY: Prey, in both cases to hold a prisoner. The implication seems to be that at this level, touching the field can be physically unpleasant.
JANEWAY: How's our wolf in Sickbay?
TUVOK: Still unconscious, behind a level five forcefield
A level three field is used by the EMH to contain a macroscopic virus in VOY: Macrocosm
EMH: It's rate of growth shouldn't hinder our analysis, as long as it's genetic structure stays the same.
EMH: Computer, erect a level three forcefield around the microscope station.
Seven of Nine erects a level three forcefield to prevent a villain from getting oxygen (or escaping) in Voy: One
SEVEN: Let's play another game. Let's imagine that the oxygen on the bridge has been depleted.
SEVEN: What do you suppose the results might be? Computer, seal the bridge with a level three forcefield.
So what can we tell from this?
Forcefields seem to be limited by the equipment used to produce them, for example, the doctor's microscope station can only erect a level five field, no matter how much he'd like to increase it. Bulkheads seem to be limited to level eight whereas the field emitters in main engineering can go up to level ten.
Higher level fields use more power and drain their power sources faster. They're prone to exploding when tampered with.
Lower level fields can be left on overnight, suggesting their power usage is low.
Fields at level three and above stop physical objects from passing. Level five fields will cause a nasty shock, level seven fields will prevent gas passage, level eight fields will block plasma fires and level nine fields can prevent even substantial explosions.
Interestingly, there appear to be a number of differing protocols around different kinds of fields. For example, biohazard fields only seem to go from one to three, with one being the highest whereas containment fields go from one to three with three being the highest, and presumably analagous to a level ten forcefield. Dampening fields go from one to four with one being the highest. It all gets a bit confusing after that.