At first glance I would dismiss it as the usual analog-digital gerrymandering.
However, one meaningful digital PLL example I can think of is in frequency synthesis e.g; use of pulse swallowing against a stable clock reference. Whenever you wiggle a VCO, VCXO, TCXO... all that's analog.
So, perhaps the right definition is whether the block is part of the boundary scan chain
M.G.Rajan
www.eecalc.com