The term “special forces” can have many different meanings. The western definition of “special operations forces” implies the ultimate operators. The Soviet definition of “special purpose forces” just means those trained for a certain role. Thus engineers and chemical personnel technically count as “special troops”, and many “SPF” are mainly recon personnel.
Training Circular 7-100.2 differentiates “SPF” and “commandos”, the former capable of things comparable to western-style SOF like training/leading allied irregulars and the latter capable of more “muscular” missions that require a bigger conventional force (commandos are listed as being able to operate at up to battalion level, something SPF never would even during direction action missions). There is a large amount of overlap, and they are more similar than different.
Then there are the “special forces” that would be considered just larger light infantry formations by the standards of other countries. Or not even that, with Stryker/BTR formations viable for (and sometimes expected to be used in) air assault operations due to their larger dismount size and some “SF” riding in APCs when operating with mechanized forces and/or facing threats that require appropriate protection.
Thus the clearest definition boils down to a vague “units with personnel trained to at least theoretically higher standards and intended to conduct tasks above and beyond those assigned to run of the mill formations”. Interestingly, I’m seeing (with the precedent of Syrian commandos in Lebanon and the proposed TRICAP Ranger/Helicopter formations) certain “special forces” being used an antitank reserve with their increased skill, more and better AT weapons than comparable line infantry formations, and the ability to be deployed quickly.