Function choice_alternatives_suffix 

pub(crate) fn choice_alternatives_suffix(
    noun: Option<&str>,
    optional: bool,
    null_word: &str,
) -> String

The other values a choice parameter accepts, after its quoted choices: , or a data frame for an Either<.., R> layer (noun, the R arm), , or <null_word> for an Option<..> layer (optional), , a data frame, or <null_word> for both, nothing for neither. Shared by the parameter’s @param line (ParamAttrs::choice_doc_suffix, which passes NULL for no choice) and the argument error of an Either choice parameter (which passes NULL), so the two word the alternatives alike.