Hi!
Those modes are only useful with very specific hardware 
Single digit clock will light up the leds 0-9. Combined with an overlay or plexiglas, this can be used to display the time one digit at a time. I didn’t like the effect that much in the long term and it was removed from WLED in version 0.9.0 (i could make it availabe as a usermod though)
Cronixie is WLED support for this nixie-style clock kit! It looks absolutely amazing, but since probably less than 1 in 1000 WLED installations use it, I will probably remove it from the core and make it available as an usermod soon.