Hello @kapil,
A split plot design is a special case of a factorial treatment structure.
It is used when some factors are harder (or more expensive) to vary than others.Basically a split plot design consists of two experiments with different experimental units of different “size”.
You can refer this for more information:https://stat.ethz.ch/education/semesters/as2015/anova/08_Split_Plots.pdf
Hope it helps!!
Thank You!!