Updating line-items

Updating line-items

The spec apears to support a PUT to an line-item endpoint to update its properties. However, the spec states:

The platform MUST NOT modify the 'resourceId', 'resourceLinkId' and 'tag' values. It MAY modify the other properties, although preserving the original values from the tool is recommended.

Is this intended to mean that a platform must not allow the 'resourceId', 'resourceLinkId' and 'tag' values even when requested to do so by the tool, or merely that it should not take it upon itself to do so?

My current experience with IMS certified platforms is that some allow these properties to be updated and some do not. Moreover some do not allow the label or points possible to be updated. What is the minimum requirement to gain IMS certification in relation to updating line-items?