What does the 'master' option in the attachment view settings stand for? I am using some attachment views to display the latest one of different content types in one view.

I was using the first (block) view as the parent view and was attaching the others to this view. This, in many cases ok. But now I ran into some styling issues, caused by the overly complicated markup.

So I decided to delete the block view, convert all the views to attachments and  of the view attachments, and attach all of them to 'master'. I was hoping this would give me a homogeneous markup.

But now the block disappeared completely. It is not even visible in the blocks admin page.

So my question is, what does the 'master' in the attachment options stand for?
The master display is the default one created automatically for every view. It's not a block, so it won't show up in the list. For views, only their block displays show up in that list.

A quick trick to get access to the Master display: edit the view then without selecting any of the displays append "/default' to the URL and hit return.