You can easily add layers to any visual to separate complexity and make it easier to edit sections of your diagram separately. This often makes sense for floor plans, HVAC diagrams, and electrical drawings.
Layers have two properties: Visible and Clickable. If the Visible box for a layer is unchecked, then all objects in this layer do not show on the screen or print in the drawing. If the Clickable box is unchecked then, even if the objects show, they behave as if they are part of the drawing background; they cannot be moved, deleted or changed in any way. Layers that are not visible are also not clickable.
When a layer is visible, but not clickable, you can add symbols to it and see the new items you added, but you can't click on them. This means the layer is locked. To be able to edit objects in a specific layer, enable the clickable function for that layer and unlock it.
You can modify layer properties at any time using the Manage Layers command under Page->Layers.
You can toggle locking and unlocking a layer using the "Clickable" property.