Nothing changes, when I remove that “+1 staff” sign – what is it supposed to do? Or what does it inform about?
Also, as can be seen in the screenshot: the barlines of the harp accolade are not drawn through the 2 system, but instead only on each system itself. This is only in Galley view (and only in my file; a simple test with a piano setup shows that barlines are drawn over both staves also in Galley View). Is there a function that would change that?
Usually it indicates that you have added a staff above or below. I note that you are using staff visibility (double click the system break signpost to see what is overridden), it could explain why you don’t see a difference when you delete the +1 signpost…
You add staves by invoking the caret on a staff, alt-click>Staff>Add staff above/below.
Thank you very much for your feedback, @MarcLarcher ! So far I have no clue where I’ve added a system (but I am not done with the piece yet).
Any thoughts on the barline “feature” and why it does not reach over the 2 harp systems in Galley view?
The number of hidden time signature signposts (the red ones) suggests that perhaps you’ve input “local” time signatures (that only apply to individual staves, including the top/bottom staves of grand staff instruments).
@Lillie_Harris Not sure why you mention the time signatures – they are not the problem. This post is about the “+1 staff” and the barlines…
Yes why indeed am I mentioning time signatures, what could I possibly know 
Those red time signature-y signposts could be indicating staff-specific time signatures, or staff-specific barlines. I strongly suspect* that staff-specific barlines won’t join across multiple staves, because they’re staff-specific. (*leaving room for doubt because I’m away from Dorico and it’s 11pm.)
If I’m just totally wrong, double-check you’re showing bracket & barline change signposts and look for an override there.
Or, since I remember you saying your imported file resulted in separate single-staff instruments for each staff of a usually-grand-staff instrument, is that perhaps the case here?
If you want a definitive answer, post the project.
Okay, you didn’t mention the implications in you previous post, so I didn’t get what you meant. Thanks for clarifying, @Lillie_Harris !
However, why would the behaviour of such a feature be different between Gallery and Page view? In Page view, barlines are fine…
The import did indeed create separate players (no idea why, but I am not going back to mxml at this point). I moved the “harp players” into one player, but that still wouldn’t allow me to add an accolade, so I changed the instrument of the … well: first instrument (first staff of the harp player) to “Harp”, which added another staff and the accolade. And here we are: the barlines do not connect in galley view.
Well, it is not important for me. (But if you guys think, that there might be a bug and you need my files, let me know.)
On a side note: Would it be possible to add an alias to the documentation that links “accolade” to “braces”? Because “accolade” does not even once appear in the documentation, which seems strange, since it is a rather important feature of notation… I had a hard time finding the proper section/entry, since “braces” is not a common term for accolades. (I am thinking of things related to teeth when somebody mentions braces…)
Are you a native French speaker, perchance? The French Dorico manual does use accolade; the English manual doesn’t. I can’t think of a single program or reference guide that in English uses accolade, as the common term in English is “brace”.
Aha, that makes a lot of sense. No, I am German speaking, but I do use the English manual. OK, need to get my thinking straight. Thanks for clarifying, @pianoleo ! 
These two view modes serve different purposes and render differently as a result. This is why staves that are hidden in page view still appear in galley view, for instance.
One could always create a new harp prat and use “select to end of flow” to copy each staff into the new part. It’s possible the separate staves and all were the result of the program that generated the file which was imported (if the file was imported).
Perhaps now that solution is impractical after you have spent so much time on your current file.
Or it’s possible that there’s a bracket/barline change signpost, or that staff-specific time signatures and barlines are preventing the barlines joining across the two staves. Without the project, it’s hard to say for sure.
How useful or important a clearer diagnosis and explanation is, is up to @rowild. “To share or not to share, that is the forum’s question.”