"auto": the default rules – one number is copied, several numbers continue as a linear series, dates and texts
with a trailing number count up, everything else repeats;
"alternate": the other choice for numbers, dates and texts with a number (⌥/Alt or Ctrl while dragging the fill
handle): one number counts up by 1, a series is copied;
"copy": always repeat the source values (Ctrl/⌘+D, Ctrl/⌘+R).
How computeFillValues continues the source values:
"auto": the default rules – one number is copied, several numbers continue as a linear series, dates and texts with a trailing number count up, everything else repeats;"alternate": the other choice for numbers, dates and texts with a number (⌥/Alt or Ctrl while dragging the fill handle): one number counts up by 1, a series is copied;"copy": always repeat the source values (Ctrl/⌘+D, Ctrl/⌘+R).