This attribute allows to define if the module can only have its mirrored position, but maintaining the original design.
With True value, the item may have its position mirrored after its insertion in the environment through the Mirror option on the right button. However, its original design will remain the same, only mirrored.
In the registration of this attribute, its default setting defines that it is automatically created (in a hidden way) for all entities (property Automatic Creation) with default value False, that is, the attribute is applied in all modules to define that modules, originally, cannot have their position mirrored only. In case you need to define that the item can have its position only mirrored, you need to apply the attribute on the module and change its value to True.
Handles with specific designs that cannot be mirrored should have this attribute applied with True value. When mirroring these handles, the position is mirrored, but the handle design stays the same.
Through the images below you can see that, on the left, with the attribute CanMirrorOnlyPosition with value False or not even applied (due to the default value - False), the left door handle is mirrored and so is its detail, mirroring the entire design. On the right image, with the CanMirrorOnlyPosition attribute set to True, the left door handle is mirrored but its design is not.