You got that right. Just to add few more detail.
‘link to source’ property will be set to ‘true’ whenever you have IS document created by various sources such as XML, XSD, Broker Publishable doc, WSDL, Flat file schema etc., All these generated documents would adhere to the schema that is attached with the created IS documents… Even if you do ‘Lock for Edit’, where you will see ‘green tick’ as if like editable, you wouldn’t be able to edit these generated IS docs. This is because, the generated document is linked to a schema & changes are not permitted in it, thus Designer doesn’t allow you to change it directly.
As you rightly found, setting ‘link to source’ property to ‘false’ will allow you to ‘edit and change document’, however it will give you a warning that, this document is no more tied with ‘schema’.
Hope this helps