  We show that unit $\mathcal{O}_{F,X}^\Lambda$-modules of Emerton and Kisin
provide an analogue of locally constant sheaves in the context of B\"ockle-Pink
$\Lambda$-crystals. For example they form a tannakian category if the
coefficient algebra $\Lambda$ is a field. Our results hold for a big class of
coefficien algebras which includes Drinfeld rings, and for arbitary locally
noetherian base schemes.
