What are the different ways to describe cost behavior?

The Cost Function

Q1: What are the different ways to describe cost behavior?
Q2: What process is used to estimate future costs?
Q3: How are engineered estimates, account analysis, and
twopoint methods used to estimate cost functions?
Q4: How does a scatter plot assist with categorizing a cost?
Q5: How is regression analysis used to estimate a cost
function?
Q6: How are cost estimates used in decision making?