Summation notation with a comma- what does this mean

Question

I am reading a document with this expression:

evidencei=∑jWi, jxj+bi

Can someone explain what this means?

In context:

We also add some extra evidence called a bias. Basically, we want to be able to say that some things are more likely independent of the input. The result is that the evidence for a class i given an input x is:

evidencei=∑jWi, jxj+bi

where Wi is the weights and bi is the bias for class i, and j is an index for summing over the pixels in our input image x. We then convert the evidence tallies into our predicted probabilities y using the "softmax" function:

Answer 1

You seem to be coming from the TensorFlow documentation, as am I. After half an hour of searching for commas in Sigma Notation, I have realized that the comma is separating the subscript $i$ and $j$ of $\mathbf W$, not any part of the whole sum.

It is a little more clear here: $$\sum_{j} \mathbb W_{i,j} x_j+b_i $$

$W_{i,j}$ is the weight for class $i$ at pixel $j$.