Now that’s a tricky one!
Some say the bees don’t like plastic foundation, others say it’s fine. I don’t know, I’ve never used it and never would. I don’t like plastic – there are always chemicals and bees have enough problems already.
Frames with nothing in them need to be fitted with beeswax foundation – either wired or unwired. Wired foundation doesn’t sag and is stronger and less likely to break in the extractor. Alternatively you could fit them with 4cm starter strips then the bees will draw wild comb.
Frames with thin wires across can be fitted with unwired beeswax foundation then the wire is embedded into the wax using either a heated embedding tool or by passing an electric current through the wires.
If your frames are deeps, they are for the brood box.
If they are shallow, they are for the supers.
Or they could be Rose hive frames in which case they will suit both.
Without seeing them I can’t be more help.

The measurements of the two types of top bar are marked in the illustration above and in the text.
Advantages and disadvantages. The fatter bars reduce the gap between frame tops at standard brood box spacing from approximately 13mm to 8mm. This narrower gap is close to a bee-space so the bees are less inclined to close it with brace comb. Whether this is an advantage or a disadvantage depends on how you view the continual build-up of brace comb – is it a nuisance to keep removing it or do you regard it as a crop?
Another thing to think about is the price. Thornes will charge 76p for the narrow top bar aka DN1 or £1.00 for a wide top bar aka (DN2) https://www.thorne.co.uk/frames-and-foundations/frames/frame-parts/bs-frame-parts
Again whether that is too much to pay depends on you and your beekeeping. Like a lot of other things it’s down to personal preference and of course your bees and the way they are.

But why…?