  The pathway(s) to seeding the massive black holes (MBHs) that exist at the
heart of galaxies in the present and distant Universe remains an unsolved
problem. Here we categorise, describe and quantitatively discuss the formation
pathways of both $\textit{light}$ and $\textit{heavy}$ seeds. We emphasise that
the most recent computational models suggest that rather than a bimodal-like
mass spectrum between $\textit{light}$ and $\textit{heavy}$ seeds with
$\textit{light}$ at one end and $\textit{heavy}$ at the other that instead a
continuum exists. $\textit{Light}$ seeds being more ubiquitous and the heavier
seeds becoming less and less abundant due the rarer environmental conditions
required for their formation. We therefore examine the different mechanisms
that give rise to different seed mass spectrums. We show how and why the
mechanisms that produce the $\textit{heaviest}$ seeds are also among the rarest
events in the Universe and are hence extremely unlikely to be the seeds for the
vast majority of the MBH population. We quantify, within the limits of the
current large uncertainties in the seeding processes, the expected number
densities of the seed mass spectrum. We argue that $\textit{light}$ seeds must
be at least $10^{3}$ to $10^{5}$ times more numerous than $\textit{heavy}$
seeds to explain the MBH population as a whole. Based on our current
understanding of the seed population this makes $\textit{heavy}$ seeds
($\rm{M_{seed}} > 10^3$ M$_{\odot}$) a significantly more likely pathway given
that $\textit{heavy}$ seeds have an abundance pattern than is close to and
likely in excess of $10^{-4}$ compared to $\textit{light}$ seeds. Finally, we
examine the current state-of-the-art in numerical calculations and recent
observations and plot a path forward for near-future advances in both domains.
