ISO15919 is an international standard for the transliteration of Indic scripts to Latin alphabet. IAST is a slightly different scheme used for Sanskrit and north Indian languages.
“sulida bāḷeya haṇṇinaṁdadi, kaḷeda sigurina kabbinaṁdadi, aḷida uṣṇada hālinaṁdadi sulabhavāgirpa kannaḍa”
“ಸುಲಿದ ಬಾಳೆಯ ಹಣ್ಣಿನಂದದಿ, ಕಳೆದ ಸಿಗುರಿನ ಕಬ್ಬಿನಂದದಿ, ಅಳಿದ ಉಷ್ಣದ ಹಾಲಿನಂದದಿ ಸುಲಭವಾಗಿರ್ಪ ಕನ್ನಡ”
“yadā yadā hi dharmasya glānirbhavati bhārata | abhyutthānamadharmasya tadātmānaṁ sr̥jāmyaham ||”
“यदा यदा हि धर्मस्य ग्लानिर्भवति भारत । अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ॥”
Let's say you received a Baraha document containing Gujarati text which you don't know how to read. If you are familiar with any other Indian script, you can choose that script in Script Override menu. But, if you don't know any Indian script, you may choose ISO15919 or IAST output in Settings, which will display the Gujarati text using Latin characters with diacritics as shown in the picture.

Baraha supports ISO15919/IAST in the following way: