  We investigate the Cauchy--Szeg\H{o} projection for quaternionic Siegel upper
half space to obtain the pointwise (higher order) regularity estimates for
Cauchy--Szeg\H{o} kernel and prove that the Cauchy--Szeg\H{o} kernel is
non-zero everywhere, which further yields a non-degenerated pointwise lower
bound. As applications, we prove the uniform boundedness of Cauchy--Szeg\H{o}
projection on every atom on the quaternionic Heisenberg group, which is used to
give an atomic decomposition of regular Hardy space $ H^p$ on quaternionic
Siegel upper half space for $2/3<p\leq1$. Moreover, we establish the
characterisation of singular values of the commutator of Cauchy--Szeg\H{o}
projection based on the kernel estimates and on the recent new approach by
Fan--Lacey--Li. The quaternionic structure (lack of commutativity) is encoded
in the symmetry groups of regular functions and the associated partial
differential equations.
