...OK, it's been far more than 24 hours, and I don't really recall what I was trying to write originally, so I'll just summarize my thoughts.
Immediate comment: piedforts aren't really common enough for their own separate category. Besides, they're pretty much just another kind of pattern anyway.
With that in mind...
Well, as I've already mentioned,
normally, piedforts are a kind of pattern, and the Numista classification reflects this.
There are some exceptions when it's not the case, however.
1) France and Israel, at least, used to regularly make yearly piedfort sets of circulating types, in Israel's case in quite significant amounts; it's hard to call such regular issues patterns - they might be more like NCLT. (Were they legal tender? I'm not sure.)
Similarly, the UK was, and still is, regularly making piedfort versions of circulating commemoratives (and even of some standard circulation coins), again in quite significant amounts; those are, yet again, more like NCLT than patterns, though, just as with the above, presumably the defining issue is whether they are/were legal tender (which might be hard to figure out).
2) Many NCLT issues are made in regular and piedfort versions, with no real reason to claim that one of the version is real NCLT and the other isn't. In this case both are non-circulating coins.
In a few Israeli cases (and perhaps also elsewhere?), there is in fact
no "regular" version of the piedfort; it might be a matter of philosophical dispute whether coins such as
this or
this really should be called piedforts at all.
3) Some very old piedforts were made back when the distinction between NCLT and patterns was even looser than it is now, and so was the distinction between circulating coins and NCLT; this means that
this type is listed as a standard circulation coin - it probably wasn't
really such, but we might not necessarily be able to tell what it
was.
A bunch of similar coins (thickened by factors other than 2) are listed as "thick strikes" (especially for Hungary).