Unfortunately, “subspecies” is probably the most controversial taxonomic category. Species (in the general sense, without some edge cases) have unique characters that distinguish them from other taxa, and higher-ranking taxa are groups of species with a common set of characters and a common (likely) origin. But “subspecies” is a something like a “some form within a species”.
The modern concept of subspecies as a taxonomic category was formulated (using birds as an example) in the middle of the last century by Ernst Mayr in a rather general sense:
geographically defined aggregates of local populations which differ taxonomically from other such subdivisions of a species
Although this concept is still more or less followed by taxonomists, it has been the subject of criticism almost since its publication. E.g. A. Landsborough Thomson (1969) The Subspecies Concept, Bird Study, 16:1, 1-13.
The broad definition allows the term to be used rather arbitrarily. Which is hardly a good thing for a scientific category. In addition, the modern view of evolution and the origin of species probably leads to a new view of the concept of species. See some details: Guiding Principles for Identifying Species and Subspecies
Anyway, Mayr’s classical geographic “subspecies” cannot have always clear unique characters. Otherwise, they would be considered as separate species. There is always (except perhaps for “island” “subspecies”) a “transitional zone” between subspecies within a species. And there are always some individuals that cannot be confidently morphologically classified as either subspecies. Or identified as both, depending on one’s point of view. Perhaps it is better to leave the identification of such “morphologically uncertain” or hybrid individuals to the species level.
Therefore, I try not to use this taxonomic category unless it is an established tradition.