Stewart Robert Hinsley writes

Bracken is a fern. Some ferns have both fertile (spore-bearing) and
sterile fronds; others, I presume, have only fertile fronds.

I thought it was more that some ferns had spore bearing fronds which
were distinct from the non spore bearing fronds (like Osmunda and
polypody) whereas others had fertile fronds which were not distinct. But
I could well be wrong on that.

(Quite possibly pteridologists can identify fern species by the
distribution and shape of the sporangia.)

It's an important character in identification from what I see in the
field guides.

--
Kay