Specification information about category ratings of tractors & implements.
A "category" is defined by horsepower, hitches and pin sizes.
Wherever a given tractor or implement's numbers fall into, is it's category.
Refer to your tractor or implements owners manual.
It will have the rated category listed in it.

The chart shows some widely accepted hp guidelines-not rules.
Some manufacturers slip above and below with regards to engine hp.
Implement hitch width may also vary slightly at the pins by manufacturers,
but will always be within close specs to the category it's in.

The pin sizes however are non-negotiable, and are category-specific.
Both tractor & implement will accept the same pin size of their category.

There are now FIVE tractor category's.
This is mostly due to sub-compacts and over-sized bread basket tractors.

Category 0
Category 1
Category 2
Category 3
Category 4

Several category horsepower specs overlap others though.
Which is makes the newer system difficult to accept for many.

The true difference isn't in the hp rating anyway.
It's in the link-pin sizes.
This is where the stress between the tractor and the implement happen.
As an implement is pulled, dragged or lifted, force is transfered at the pin.
So pin size must be adequate to take the load of a given category rating.

Buying or attaching any implement to a tractor:
Determine the tractor or implement's category rating.
Never mis-match an implement and tractor category for use.
Serious or fatal injuries may result as well as damage to the tractor and implement.

NOTE:
If you are looking for a tractor and are not expierienced, be aware:
ENGINE, PTO & DRAWBAR HP ARE VERY DIFFERENT RATINGS!!
Engine horse power is always higher than drawbar horse power.
Engine hp testing is just the engine power output.
Drawbar hp testing is to see how much can get used to pull something.
PTO hp testing determines what load can be handled at the output shaft.

There are reasons to test for all.
One person may be interested in using the engine to run a stationary mill,
Another may be interested in skidding logs or moving rocks.
Someone else may be interested in PTO hydraulics or water pumping.

Over or under spending by using the wrong rating could be disastrous.
Whatever your need, you'll be much happier buying enough tractor to do the job right the first time, without over-spending.