When you have a fractional design, some of the effects are confounded with each other. That is, you cannot estimate all of the effects separately. For example, if Factor A is confounded with the three-way interaction BCD, then the estimated effect for A also includes any effect due to the BCD interaction. Effects that are confounded are said to be aliased. The alias structure describes the confounding that occurs in a design.

For the water resistance data, the design is a full factorial not a fractional factorial design. Therefore, there is no confounding so each effect in the model can be estimated separately.