What is "May be Changed" Flag in Attachment Form ? How to Disable "May be Changed" Flag in Attachments

The ‘May Be Changed’ flag is not updatable unless you add the Attachment as a Standard document. 

In Other Words if the Attachment is Selected from a Published  Source in EBS, That Cannot be Updated.

When adding the Attachment (Paper Clip icon) 
 – click on the Document Catalog button. 
 – Select the Standard Document from the list of values in the Category field. 
The ‘May Be Changed’ flag will be unchecked and Users will not be able to update the Attachment.

If there are no Document catalog for the Category field, then can add one attachment and click "Publish to Catalog" then it will prompt to select usage type "Template" and "Standard".

-> For standard usage type this 'may be changed' check box is always disabled.
-> For One time and Template usage types this may be changed check box is
always enabled.