A value of your own type, as the browser gets it in a cell: what the cell shows (formatted by the column's type), what a copy and an export take, and what a filter of the browser compares.

Masu knows numbers, decimals, dates and times, booleans and text, and a struct that implements String.Chars. For any other struct, implement this protocol: a rich text that shows its text, a range of days that shows 2027-03-15/2027-03-20.

defimpl Masu.Wire, for: Kotoba.Content do
  def to_wire(content), do: content.text
end

defimpl Masu.Wire, for: Koyomi.DateRange do
  def to_wire(range), do: "#{range.from}/#{range.to}"
end

The value goes to the browser as JSON: give text, a number, a boolean, or nil. An editor of your own (see the Editors of your own guide) gets the value as the row has it, not this.