Your original question was about how to isolate an attachment/slot for boning, I assume by boning you mean bone creation, therefore what you need is to select the Create tool.
You can read more about it here in the user guide:
Tools - Spine User Guide: Create Tool
You can also see this in action in this video at 6:34 :
Spine User Guide - Tools (2/2) - YouTube
You need a parent bone because you are creating a new bone, which requires to always specify which bone is its parent first. Next you are allowed to choose one or more images, and when you hold Ctrl and clock on one, the image is outlined, which makes creating the bone easier because you can see the silhouette no matter what is covering the image.
I hope this is a bit clearer 😃
You can also deactivate a bone and all of its children visibility by right-clicking a bone's visibility dot in the tree. This can also greatly speed up clearing or restoring the area around an attachment.
