To expand on what rmix22 says:
Presumably, your function describes some sort of physical process or phenomenon (you haven't given us many clues!), and each of the five parameters has a plausible range of values. In the absence of any understanding of the problem, mathematically, we can only assume that each of the parameters is valid for any value from negative infinity to positive infinity, but when modelling physical phenomena, the "valid" range is often much smaller. The mathematical solution could be visualised as a possibly very complex multi-dimensional "surface" with possibly many local maxima and minima (possibly an infinite number?), depending on the values of the 5 parameters, but if any or all of the parameters has a small valid range, the solution space could be much simpler, with only a small number of local maxima and minima; maybe only a single solution.
Consider a much simpler problem - find the maximum value of the expression:
z(x,y) = x^2 + y^2
By inspection, this function tends to infinity as either x or y approaches plus or minus infinity. However, if the valid ranges of x and y are [0.0 to 1.0] say, then there is only one maximum value that we are interested in:
z(1,1) = 2
Or an even simpler case: what is the maximum value of the following function?
y = sin(x) * x^2
There are an infinite number of local maxima and minima, each one bigger than the previous (as x increases), but if x lies in the range [0 to pi], there is a unique maximum vale of interest.
Hope this helps!
