  In the context of positive infinite-dimensional linear systems, we
systematically study $L^p$-admissible control and observation operators with
respect to the limit-cases $p=\infty$ and $p=1$, respectively. This requires an
in-depth understanding of the order structure on the extrapolation space
$X_{-1}$, which we provide. These properties of $X_{-1}$ also enable us to
discuss when zero-class admissibility is automatic. While those limit-cases are
the weakest form of admissibility on the $L^p$-scale, it is remarkable that
they sometimes follow from order theoretic and geometric assumptions. Our
assumptions on the geometries of the involved spaces are minimal.
