Clarification: Why the gauge symmetry of pure Yang-Mills is $PU(n)$ and not $SU(n)$? I am quoting the following from the Wikipedia article on the projective unitary group:

In the pure Yang–Mills $SU(n)$ gauge theory, which is a gauge theory
  with only gluons and no fundamental matter, all fields transform in
  the adjoint of the gauge group $SU(n)$. The $Z/n$ center of $SU(n)$
  commutes, being in the center, with $SU(n)$-valued fields and so the
  adjoint action of the center is trivial. Therefore the gauge symmetry
  is the quotient of $SU(n)$ by $Z/n$, which is $PU(n)$ and it acts on
  fields using the adjoint action described above.
In this context, the distinction between $SU(n)$ and $PU(n)$ has an
  important physical consequence. $SU(n)$ is simply connected, but the
  fundamental group of $PU(n)$ is $Z/n$, the cyclic group of order $n$.
  Therefore a $PU(n)$ gauge theory with adjoint scalars will have
  nontrivial codimension 2 vortices in which the expectation values of
  the scalars wind around $PU(n)$'s nontrivial cycle as one encircles
  the vortex. These vortices, therefore, also have charges in $Z/n$,
  which implies that they attract each other and when $n$ come into
  contact they annihilate. An example of such a vortex is the
  Douglas–Shenker string in $SU(n)$ Seiberg–Witten gauge theories.



*

*What is the center of $SU(n)$?

*What does it mean the adjoint action to be trivial? Which action are they talking about?

*I struggle to understand why the resulting gauge symmetry is $PU(n)$.

*What are the "codimension 2 vertices" of the scalars?

*Does this apply for pure $SU(3)$ QCD?

*References?

 A: $SU(N)$ is the $N$-fold cover of $PSU(N)$. They share the same Lie algebra, so the Yang-Mills action would look identical locally. The center of $SU(N)$ is just $Z_N$. At the level of representations, the fundamental representation of $SU(N)$ is a projective representation of $PU(N)$, and only the adjoint ones are linear representations of $PU(N)$. 
If the matter fields all transform in the adjoint representation, then it makes sense to say that the gauge group is actually $PU(N)$. A simple explanation is that by taking tensor product of adjoint representations you never get the fundamental ones, so the Hilbert space is restricted.
Because $PSU(N)=SU(N)/Z_N$, the global topology of $PU(N)$ is nontrivial. For example, the fundamental group $\pi_1(PU(N))=Z_N$, so there are nontrivial "vortex lines" in the scalar matter field, around which you pick up a holonomy in the center $Z_N$. These topological excitations themselves are one-dimensional objects, and have "codimension" 2.
Quarks in $SU(3)$ QCD transform as the fundamental representation.
