As you open and edit documents in the various document editors, you should note the following document editing concurrency rules:
If a document is edited in one document editor, it cannot be modified in any other document editor or the Library or deleted. For example, if you have a web page open in the Web Page editor, you cannot rename or delete that web page from the Library. Or, if you edit a module in one Outline Editor tab but that same module is open in another Outline Editor tab, you can edit the module only in the first Outline Editor tab in which you initiated editing. A "Document in Use" message is displayed in any such document editing concurrency scenario.
Editors other than the Outline Editor (such as Web Page, Package, and Topic Editors) lock the document as soon as it is opened. Once a document is locked it cannot be changed until you close the editor.
The Outline Editor is different from the rest of editors in that it edits multiple module/section documents. Note that, from this perspective, the root outline document is co-equal with the other module/section documents in the outline. Since a section can be part of multiple outlines, the same section can be present in multiple Outline Editors simultaneously.
The Outline Editor does not lock the module/section documents in the outline when they are displayed or expanded. It locks them before it executes the first operation that modifies them (add or remove links to it, rename it or do anything that actually changes the document). If you create a section in an Outline Editor, the new document is created but it is not locked, but as soon as you insert something into the new section, it is locked. If you unlink the new section from the module, the document still remains locked because you can undo the unlink and then undo the changes you made to the document.