Whilst some of the riders start off in the RDA there is a difference between Para and RDA, for further information on the RDA please use this link RDA

In  Dressage   rider and horse perform a series of set movements in a marked arena they are judged and marked on the same principles  as ablebodied, There are three classes in which PE riders can compete:
• individual – riding a set  test
• Freestyle – riding an individually choreographed performance which includes set movements to music.
• team - nations compete against each other using their best 3 riders. 
The tests ridden vary according to the rider’s PE Grade and that is determined by the significance of their physical or visual impairment.

Para dressage riders & the Classification Process
Para Dressage riders are like any other world-class athlete.  They are determined and committed to their sport, and may have to overcome enormous physical and mental obstacles to compete against the best in their field  both disabled and able bodied.
Para Equestrians have many different types of impairments or disabilities.  Fair competition requires that people with similar levels of impairment  compete against each other and ride tests suited to their level of impairment.    The most disabled athletes (Grade Ia) ride tests in walk only; the least disabled (Grade IV) ride tests of medium (M) standard.

There are four Grades, each with its own group of physical and visual functional profiles.  The competitor’s mobility, strength , coordination or eyesight are assessed by an FEI PE Classifier in order to establish their functional profile. This process is known as PE Classification.
People with profiles of similar functional ability level are grouped into four competition PR Grades: 
Grade Ia and 1b - Mainly wheelchair users with poor trunk balance and/or impairment of function in all four limbs or no trunk balance and good upper limb function.
Grade II - Mainly wheelchair users or those with severe physical disability involving the trunk and with mild to good upper limb function, or severe unilateral impairment.
Grade III - Mainly able to walk without support, with moderate unilateral impairment, moderate impairment in four limbs or severe arm impairment. May require a wheelchair for longer distances or due to lack of stamina. Total loss of sight in both eyes.
Grade IV - Impairment in one or two limbs or some degree of visual impairment.

When necessary, riders can use compensating aids approved by FEI PE, such as a special saddle, adapted reins, elastic bands, two whips etc.  These can be used when they compete in both able bodied and Para Equestrian events.
The competition test for each Grade is compatible with the functional ability of people with the same potential.
Grade Ia: Walk only
Grade Ib: Walk and some trot
Grade II:  Walk and more complicated trot pattern; canter in freestyle
Grade III: Walk, trot and canter
Grade IV: Walk, trot, canter including lateral work and medium paces