Examples

:Tab /,

hello , there
hi    , you

:Tab /,/r0

hello,there
   hi,  you

:Tab /,/r1c1l0

# the separator counts as a column
# [r]ight align, [1] space, [c]enter align the comma, [1] space,
# [l]eft align, [0] spaces
hello , etc
   hi , etc

:Tab /^[^,]*\zs,/r0

abc,hello
  c,hi there
  a,yo

Specifiers

l|r|cN    # left-right-center, with N spaces padding (l0)
r1c1l0    # multiple specifiers (one per column)
          # the separator counts as a column