In a number of textbooks, and on the wikipedia page for natural classes, I see that "class" and "natural class" are used interchangeably, see excerpt below (with key words in bold)
In phonology, a natural class is a set of phonemes in a language that share certain distinctive features. A natural class is determined by participation in shared phonological processes, described using the minimum number of features necessary for descriptive adequacy.
Classes are defined by distinctive features having reference to articulatory and acoustic phonetic properties, including manners of articulation, places of articulation, voicing, and continuance. For example, the set containing the sounds /p/, /t/, and /k/ is a natural class of voiceless stops in American Standard English. This class is one of several other classes, including the voiced stops (/b/, /d/, and /g/), voiceless fricatives (/f/, /θ/, /s/, /ʃ/, and /h/), sonorants, and vowels.
In essence, I just wonder what the modifier "natural" means in front of "class". Is there a distinction between a "class" and a "natural class"?