In estimating a multiple regression model, does it matter if the model is linear in

parameters?Variables?Or both linear in parameters and variables?

Yes, it does matter. If the model is linear in parameters, then the estimated coefficients will be linear functions of the independent variables. If the model is linear in variables, then the estimated coefficients will be constant. If the model is linear in both parameters and variables, then the estimated coefficients will be linear functions of both the independent variables and the parameters.