Layer - When a layer is locked, all elements on the layer are locked.
Individually - Elements can be individually locked independent from the locking status of their layer.
How to Identify Locked Elements
Locked elements cannot be modified (moved, deleted, resized, etc.) in any way.
When selected, a locked elements' hotspots and overall appearance will appear ghosted compared to unlocked element.
When hovering over an element, the information dialogue will indicate whether the element or the elements' layer is locked.
How Elements Become Locked
Predefined Layer Combinations - Office templates can be setup in a way where certain layer combinations lock and/or hide layers. Doing so prevents elements from being placed on the wrong layers or existing elements from being unintentionally modified.
Intentionally - Users may intentionally lock certain elements to prevent them from being accidentally modified.
MEP Add-on Use - Whenever someone using the MEP Add-on joins a project, all objects with an MEP component become locked and only accessible to the user with the MEP Add-on.
How to Unlock Locked Elements
Layer - Open the Layers dialogue box and click the unlock button.
Individually - Select the locked element and go to Edit > Locking > Unlock.