I have two apparently identical specimens of native grass growing out back. One is labeled

Pony tails
Stipa tenuissima

While the other is labeled

Feather grass #1
Stipa spp

A quick Googling seems to suggest that "spp" is the abbreviation for species. Then does the latter label suggest that its writer simply didn't know the species ans so substituted "spp" out of sheer laziness

I love the common names of plants; Jack-in-the-pulpit and Queen Anne's lace, baby's breath and brown-eyed Susan and Johnny-jump-ups, cigar plant and Canterbury bells. Common names do tend to be less accurate since one species may have several common names and vice versa but holodiscus discolor simply doesn't sound as nice as ocean spray.

...trying to comprehend the "sheer laziness" of the poor man who labelled every sprig of grass in your backyard... (^_^)

ssp is (also?) short for subspecies. Hybrids and custom-bred varieties of a single species are often not (rarely/never?) given individual species names as there is no such "natural" recognized species. Such items are often labelled "Genus-name ssp", or "Genus-name (species-name) var. tradename-made-up-by-inventor" (var for variety) in recoginition of the fact that it is genetically impure or exhibits characteristics not found in the wild.

Guys: Thank you for that. myr: how did you happen to know

If it was though an exhaustive Gsearch, thank you profusely for your diligence

I conclude, then, it is possible that the abbr is sometimes used where the seller either doesn't know the specie or won't spend the time to look it up