On load tap changer controller switching
On first look, it seems to be some kind of amusing mathfail from the manufacturer. The "1-n" appears in their manual as well.
For comparison an ABB tap-changer uses n-1 and n+1 its manual; same thing in another ABB manual.

I've found however the brochure of another Chinese product, but by a different company, and this one uses arrows, not minus/dash between 1 and n:



Besides making fun of the Chinglish on the faceplate, this one is perhaps less nonsensical than "1-n". The arrows notation could be taken as some kind of increment/decrement notation like --n and n++ would be in programming. Or perhaps (more likely) the arrows indicate going from tap position 1 toward tap position N (i.e the max tap number) and vice-versa.