Found it in PSE guidelines!
Quote:
Extremely Minor Fault
Gum skips or short gumming on NH stamps
Minor natural gum bend
Tiny natural paper inclusion not visually distracting
Guideline on perf tips of one side
Natural surface wrinkle on the face of a rotary press stamp
Vignette significantly shifted on bi-color stamps
Corner perf tip crease
Tiny, light toned spot
One shorter perf, about half normal length