* Written: You would typically spell out "ten" when writing in a formal context, like a letter or essay.
* Numbers: You would use the numeral "10" when writing in a numerical context, like a spreadsheet or a math equation.
Here are some examples:
* Formal writing: "There were ten people at the party."
* Numerical context: "The score was 10-7."
Ultimately, the best way to decide whether to spell out "ten" or use "10" is to consider the context and what will be most clear and appropriate for your audience.