Thank you, Dave! I completely forgot about Styles window. 😳

It's very possible that the thumbs down on Dave's post is from my fat thumb, it happens to correspond with with were I scroll when reading on my phone. As I'm unable to give it a thumbs up it would seem to be my fault.
So having come clean about that I just like to add....
It can be helpful with some models to create separate files for individual parts and combine them in one grand plan model.
So you create the different rooms of your apartment as components, using the copy/paste in place method described by Dave. This lets you work on them in detail without the rest of the model getting in the way and you can reload them into the complete model when needed. Handy when you think of it as the full model being a city and the components each house.
Combining that with layers and section planes and anything is possible.

If you add a section-plane inside a group or component [by editing the 'container']... then it only cuts what is in that context - in this case just the walls.
If you add a section-plane in the model itself it will cut everything.

Also, there is a shortcut for creating a new layer while you are inside the Entity Info window.
You can just type the new layer name inside the Layer: field and press enter to move the selected entity into the new layer without having to create the new layer first.

Just us jiminy and Dave suggested:
A best practice is to draw all your individual entities on Layer 0 and leave them there. Only after you turn individual entities into groups or components you should associate those groups or components with different layers.

When you look at the Layers window the radio button should always be on for the Layer0 and never on for any other layer. Just use the visibility check boxes to enable and disable a particular layer visibility.

File deleted.