Focus on Species
The focus here is on species. Subspecies and varieties may be mentioned in the notes, but are otherwise ignored. In the temperate fern world a majority of subtaxa seem undifferentiable at the level of the naked eye or the hand lens.
Cultivars are ignored. Several, maybe even a dozen, hardy ferns have had numerous selections made. You may still be able to identify a cultivar using the information here, but some cultivars are a stretch. If you want to identify a cultivar or selection, a good starting point is Martin Rickard's book; see the bibliography.
Sterile hybrids are ignored. Many of the species in the temperate world are actually hybrids passing as species, but the ones of interest here are restricted to those that propagate themselves in a more or less normal manner, not by laboratory culture.
Focus on Hardy
Hardy, initially at least, is defined as those ferns which can withstand a winter minimum of -15ºC or about USDA Zone 7. Why Zone 7? Zone 8 would encompass most of the temperate world, but is more work. (Zone 9 stradles the sub-tropical.) Some relaxation/expansion is expected.
Focus on Cultivation
There are far more hardy fern species in the world than there are in gardens. Most of the temperate ferns of North America and Europe are in cultivation, but most of the ferns of the Southern Hemisphere and of the Himalayas are not. Many eastern Asian ferns are only in cultivation there, not accounted for here.
Do you have a fern that meets these criteria and ought to be here? Add it.
The resources used in selecting species for inclusion here are listed in the bibliography.