  The positive operator valued localizations (POL) of a massive scalar boson
are constructed and a characterization and structural analyses of their kernels
are obtained. In the focus of this article are the causal features of the POL.
There is the well-known causal time evolution (CT). Recently a POL by Terno and
Moretti, which is a kinematical deformation of the Newton-Wigner localization
(NWL) and belongs to the here fully analyzed class of finite POL, is shown by
V.Moretti to comply with CT. A further POL with CT treated here, which is in
the same class, is the only one being the trace of a projection valued
localization (like NWL) with CT. - Causality imposes a condition CC, which
implies CT but is more restrictive than CT. Extending Moretti's method it is
shown rigorously that the POL of the class introduced by Petzold et al. satisfy
CC. Their kernels are called causal kernels, of which a rather detailed
description is achieved. One the way there the case of one spatial dimension is
solved completely. This case is instructive. In particular it directed Petzold
et al. and subsequently Henning, Wolf to find their basic one-parameter family
$K_r$ of causal kernels. The causal kernels are, up to a fixed energy factor,
normalized positive definite Lorentz invariant kernels. A full characterization
of the latter is attained due to their close relation to the zonal spherical
functions on the Lorentz group. Finally these considerations discharge into the
main result that $K_{3/2}$ is the absolute maximum, viz. $|K|\le K_{3/2}$ for
all causal kernels $K$.
