It is something from https://mathematica.stackexchange.com/questions/114634/simplifying-polynomials-with-conditions-on-variables, that intend to minimize the number of variables. The problem is the usage of ``Solve". I know how to solve equations, like

Solve[x^2 + a x + 1 == 0, x]

However, for

Solve[pol == p + q + r - b && a + b == p + q + r, pol, {b, q, p, r}]

I am confused about "pol" and its position ``Solve[pol == p + q + r - b && a + b == p + q + r, ? , {b, q, p, r}]". I can not find any similar examples like this in the help document. How does it work? Is "pol" an expression or a variable? May I have more examples?

It is also posted at https://mathematica.stackexchange.com/questions/289911/a-specific-usage-of-solve-function