From PTC KB:
Use the MASSPROP statement to update mass properties each time geometry changes. After you have specified parts or assemblies for which mass properties are to be updated, you can request the current value of a required parameter through the relations mechanism.
To update mass properties, use the following format:
MASSPROP PART NAME ASSEMBLY NAME END MASSPROP
Note: When specifying the model for which mass properties are to be calculated, enter the model name without an extension.
The MASSPROP statement can contain the IF... ELSE clause. If you add a condition to the MASSPROP statement, the mass properties of an object will be calculated only if that condition is met.