Answer:

The clad /overlay thickness entered in the Component Properties dialog box is considered as a corrosion allowance.

Example 1:

Observe that a clad thickness of 5 mm is entered in the"Clad / Overlay Thk"  box below.

 

The clad thickness is considered as 5 mm corrosion allowance as shown in the report:

Example 2:

If you have clad thickness of 3 mm in shell component (Tube Side) and also has the corrosion allowance of 1.5 in HX Properties then software considers both as a corrosion allowance. 

In this case Software sum the clad thickness and corrosion allowance together (3 mm + 1.5 mm = 4.5 mm)