When you have a rating on both a left and right limb, any combination of arms and legs, the VA adds a 10% bonus to the combined value of just those paired ratings, before folding the result back in with everything else. This is the bilateral factor, defined in 38 CFR §4.26, and it exists because a disability affecting both sides of your body limits you more than the same two ratings would if they were both on the same side.

For example:

A 20% rating on your left knee and a 10% rating on your right knee combine to 28% (20 + 10% of the remaining 80). The bilateral factor adds a 10% bonus to that combined figure: 28% × 1.1 = 30.8, which rounds to 31%. That 31% is then combined with the rest of your ratings normally, instead of the un-bonused 28%.

See the full worked example of a bilateral pair combined with a third, unrelated rating for how this plays out in a complete claim.