The only format here that requires bleed
Everything else covered on this site refuses bleed. This one demands it: five millimetres on the top, the bottom and the outside edge, but nothing on the inside edge at the fold. So a 170 by 250mm page becomes a 175 by 260mm document, and if you set your document to the trim size you will be five millimetres short on three edges.
This matters more than it sounds because of how people learn. Anyone who has ordered a Newspaper Club tabloid has internalised the rule that Newspaper Club takes no bleed, and it is a perfectly reasonable thing to have internalised. Applied here it produces a file the press cannot use. Bleed is a property of the format, not of the company, and this is the format that proves it.
The asymmetry is worth a second look too. Bleed on three edges and not the fourth means your document is not simply the page plus five millimetres all round. Set it up wrong and your artwork will be centred in a box that is the wrong shape, which pushes everything off by two and a half millimetres in one direction.
Multiples of eight, minimum sixteen
The page count rules are unlike anything else here. Every other format runs in fours, or twos on the traditional broadsheet. This one runs in eights, and it will not print fewer than sixteen pages at all.
That is a real constraint on what the format is for. A four page mini does not exist. If your content runs to twelve pages you are ordering sixteen and finding four more pages of material, and the next step up after that is twenty-four rather than twenty. Plan the content around those numbers from the beginning, because the jumps are large.
What the size is actually good for
At 170 by 250mm this is close to a paperback book or a large zine, and that is the right way to think about it. It is the only format here you would comfortably read on a train or post in an ordinary envelope, and the sixteen page minimum means it always arrives with some substance to it.
The natural uses are the ones where a full sized newspaper would be unwieldy: poetry collections, art zines, conference programmes, product lookbooks, wedding papers meant to be kept rather than read once. The small page also means far fewer columns. Two or three is right; five would give you lines too short to read.
Setting it up correctly
Set the document to 175 by 260mm, not 170 by 250mm. Place your trim guides five millimetres in from the top, bottom and outside edge, flush with the inside edge, and then your safe area guides a further ten millimetres in from the trim on those same three edges. Anything that must be read stays inside that innermost box, which works out at 160 by 230mm. Backgrounds and images that are meant to run off the page extend all the way to the document edge, which is what the bleed is for.