They are part of a system or frame break. You change the space size in the properties panel.
Jesper
I would think it is a bad solution as in “this never happens on a real score”. I might be wrong, but I really think this should only be used with frame breaks, not for a system inside a frame (with other systems having different rastral sizes).
Take a look at almost any score by R. Strauss.
Thanks for your comment. Could you please put a link to a score from Strauss that has different spacing sizes inside the same page? I’ve just spent some time checking out, and could not find one.
Don Quixote, Salome (actually any opera published by Fürstner) and Ein Heldenleben, to name a few.
All on IMSLP
I’ve just had a look at Don Quixote, and I don’t understand… Whenever there are two systems on the page, they share the exact same rastral size (which is exactly what I was saying). I don’t think you’ve understood what I meant (or I missed something, which is highly possible).
Here are three pages from Salome. Each system in each page has staves that do share the spacing size. (I’ve zoomed and measured them to be sure)
I think I did misunderstand - I thought you meant that the rastral size could not change from page to page, which is demonstrably not the case.
However, look at Don Quixote, page 67. Although the rastral size does not change, there are notes in the winds that are smaller than the rest of the orchestra. These are not cues or optional notes; the engraver had to reduce the size to be able to fit them on the page.
Ok, I will check out on that page, but I feel you got my point 
Spacing size changes should only happen with frame breaks, not system breaks (in the middle of a page).
Agreed