in the paper "displays of a formal p-divisible group" Zink defines some objects and calls them 3n-display. a 3n-display over $R$ is a quadruple $P,Q,F,F^1$ such that $P$ is a projective $W(R)$ module Q is a submodule and $F:P\to P,F^1 :Q\to P$ are functions satisfying some conditions. I don't know why these things are called $3n-display$ and I don't see any relation to $3$ or $n$ in the definition. does someone know the reason behind this name?