Meaning of declaration float ***c

I am passing an example of a spheroidal harmonic equation from C to C++ from the following document. In doing this I am facing a problem of a declaration I have never seen float ***c. The example can be seen in the same source at page 777. Specifically the function is the one below:

void solvde(…, float **y, float ***c, float **s); 

I saw some old posts explaining some not usual declaration. Also this post was somehow useful but that did not give me a full understanding of what I am looking for. I have been researching a lot but didn't find any good explanation. Additionally on the same source provided there is no explanation for that.

Can someone explain or give a small simple example of this kind of declaration and how to use it, and, most importantly, what would be its equivalent in C++?. Thanks for pointing in the right direction.