Cost models use three types of parameters to calculate project cost:
Input parameters
Reference parameters
Hidden parameters
The TotalCost parameter
Input parameter A parameter whose value you can change directly
in the tabular area. Its record lies on a blue background. A small triangle
in the upper left corner of the Value
field — 
— indicates that you can click the field
and edit the value.
Input parameters can be:
NOTE: When you change the values of input parameters, you change them only for the project in hand. The values set in the cost model do not change.
Reference parameter Parameters
that give background information about factors that take part in the calculation
of the project cost. These parameters only display their values which
you cannot change directly. Reference parameters are highlighted in gray
in the tabular area and in the cost
model. To edit a reference parameter, open the cost model and edit
the parameter there.
Hidden parameters do not appear in the Cost Estimator tabular area. You create them in the cost model, and then set them to Hidden. Their values and expressions can be edited only while editing the cost model.
This is the parameter that calculates the final
cost of a project and appears automatically at the bottom of cost models.
It is the only parameter whose type you cannot change: it is always a
reference parameter.
============================================
Creating, editing selection tables