The DGLAP equations read

$$\frac{\partial f_i(x,\mu^2)}{\partial\ln\mu^2}=\sum_j\int^1_x\frac{dz}{z}P_{ij}(z,\alpha_s(\mu^2))f_j\left(\frac{x}{z},\mu^2\right),$$

where the $f_i$ are the parton distribution functions (PDFs), $P_{ij}$ are the so-called splitting kernels and $x,z$ are longitudinal momentum fractions.

But what is $\mu$? In https://arxiv.org/pdf/hep-ph/0409313.pdf on p.26 *John Collins* says it is the renormalisation scale, which enters the PDF via dimensional regularisation. But I have already seen other authors claim that it is a factorisation scale, e.g. https://arxiv.org/pdf/hep-ph/0703156.pdf on p.2.

So, which one is it?