When setting a part as active means currently modeled part only editable. This prevents accidental modifications to other parts.
How to:
Select the part you want to activate, then right-click and choose [Set as active] from the context menu.
The name of the active part will be displayed in blue in the Scene Browser.
The blue color is light, so please check carefully.
How to deactivate
To deactivate, select any part in the Scene Browser, right-click, and choose [Deactivate All].

You can perform the "Deactivate All" operation from any part, not just the currently active one.
Optional Settings:
When a specific part is set as active, you can make all other non-active parts appear semi-transparent.
[1] Open [Option] of 3D Scene.
[2] Go to [Interaction], then check [Don't display other shapes transparently when editing features of the current shape] in the [Feature Behavior].
- Check: Non-active parts will be displayed normally (not transparent). (This is the default in IRONCAD 2023)
- Uncheck: Non-active parts will be displayed as semi-transparent.
Example when unchecked: The left part is active, and the right part is inactive.

Pressing the shortcut key [Ctrl + Shift + A] will set the selected part as active.
If you use the shortcut, be careful—it's easy to unintentionally continue working without noticing the part has been activated.
If you activate a part by mistake, press [Ctrl + Shift + A] again on the active part or follow the steps in How to Deactivate to correct it.