When you print a pdf file made from a Booklet Cover or Booklet Insides you might see recommended settings appear below the Print dialog box.
There are many possible printer settings, page sizes and orientations that affect the printed pages. An exhaustive review of these is beyond the scope of these Helps.
Here are some considerations that can help you think through your situation:
In the Page Size topic, compare the sizes of the A4 and A5 pages. Then, compare the sizes of A5 and A6 pages. Notice that in each case, one dimension matches the other page size, and the other dimension is essentially twice as large.
In the most common case, the book is laid out as a collection of A5 pages, two of which are printed side by side on an A4 page. The printer should be set to print A4 pages in landscape mode, using options to print on both sides of paper and flip on short edge (or short edge binding). Your printer driver might calls this arrangement something else.
When you look at the preview of, for example, Booklet Insides, what you see is what Bloom is actually going to try to print.
So for A5 paper size (in the final book), you see a series of A4 pages in landscape mode. Bloom wants the printer to print the first page on one A4 sheet in landscape mode, just like on the screen, without trying to do anything special. Then the second page on the back of that, flipping the paper on the short end so all the pages are the same way up when you fold it in half and flip through them. Then we want the printer to print the third preview page, then the fourth on the back of that, and so forth. So it is just like printing an ordinary A4 document in landscape mode double-sided. We do not want the printer to shrink it or do anything tricky (except double-sided), just print what you see in the preview exactly as Bloom sends it.
When you print a pdf file made from Simple, you do not see any sample set-up information. No special printer settings are usually necessary.