What is this notation with an un-sandwiched comma in the subscript?

I have a scalar deflection potential (in the study of weak lensing) and in the book (Schneider, Kochanek and Wambsganss’s Gravitational Lensing: Strong, Weak and Micro) I have the following passage:

If a source is much smaller than the angular scale on which the lens properties change, the lens mapping can be linearised locally. The distortion of images is then described by

$$
vec{A}(vec{theta})
=frac{partialvec{beta}}{partialvec{theta}}
=Big(delta_{ij}-frac{partial^2psi(vec{theta})}{partialtheta_itheta_j}Big)
=begin{pmatrix}
1-kappa-gamma_1 &gamma_2
gamma & -kappa+gamma_1end{pmatrix}$$

where we have introduced the components of the shear $gamma=gamma_1+igamma_2=|gamma|e^{2iphi}$, $$gamma_1=frac{1}{2}(psi_{,11}-psi_{,22}),$$ $$gamma_2=psi_{,12}.$$

So there it is in context. Could someone please explan the notation used in the scalar potentials on the last 2 lines?