The concept of a unipotent algebraic group over a field has been extensively studied and is fundamental in algebraic geometry. However, has the notion of a unipotent group scheme over a general base scheme been studied? Both Demazure-Gabriel and SGA3 appear to only consider the case of a base field.

I am even uncertain about the definition of these objects. For a $\mathbb{Q}$-scheme $S$, I believe that a unipotent $S$-group scheme should be a (finitely presented?) group scheme over $S$ admitting a normal series with $\mathbb{G}_a$-factors. However, I am unsure about the definition for more general bases.