The link shared in step 2 of this exercise shows latin-ext and vietnamese rules. What are these rules and why do we only need the latin rule set?
Some typefaces in the Google Fonts directory support various character sets. For example, the latin rule set that you were instructed to select in this exercise supports most Western European languages (Spanish, French, German, etc.).
The latin-ext rule set supports many Eastern European languages and the vietnamese rule set supports the Vietnamese character set.