accessibilitySpeechSpellOut asks VoiceOver to speak the sequence of characters. Can be useful for things like promo/reference/authentication codes, phone numbers... it makes more sense to announce each character rather than words and big numbers.

An app shows a reference code and it shows how VoiceOver would announce it by default saying

Example code in the image:

let codeLabel = UILabel()
let attributedLabel = NSAttributedString(
    string: "BAC1234567D",
    attributes: [.accessibilitySpeechSpellOut: true]
)
        
title.accessibilityAttributedLabel = attributedLabel