The unit plot provides a standard and reference condition to define and determine values for soil erodibility factor.
The unit plot condition is a reference condition for all of the other RUSLE2 factors.  A value for another RUSLE2 factor besides r or k is the ratio of soil loss for the given condition to the soil loss from the reference condition of the unit plot.
The starting in RUSLE2 is the product of rk, which gives a soil loss value for the reference unit plot condition.  The other factors adjust for the difference between field conditions and the reference unit plot condition.

Tons/Acre/Year  from a specific slope (L)

Unit Plot = 72.6’ and 9%
R is computed from weather records
K is a measure of erodibility for a  CONTINUOUS CULTIVATED
      FALLOW condition.
L  (72.6’) = 1
S (9%) = 1
C ( CONTINUOUS  CULTIVATED FALLOW) = 1
P (UP & DOWN) = 1

RUSLE estimates erosion when there are adjustments to the unit plot.