A word that has the letters e and o as the first among its vowels, gets also the designation of vṛddham,
As गोनर्द is a country of the east, it is a vriddham word and we have गोनर्द + छ् = गोनर्दीयः; similarly एणीपचनीयम्, भोजकटीयः.
The word prak in the sutra has been differently interpreted. The Grammarian Kunin takes the word prak as an epithet qualifying the word acharya, 'professor' understood. In that case the sutra will be thus translated:- 'In the opinion of the Eastern (or ancient) professors, a word that has the letters e and o as the first among its vowels, and is the name of a country, is called vriddham.' Under this interpretation, this will be an optional rule only, giving the opinion of eastern or ancient grammarians.
Others however take the word prak as qualifying the word desa and the meaning then is as given in the text.
Why do we say 'having the letters ए and ओ'? If a simple vowel is first among the vowels, of a word, that word will not be called vriddham: and as such will not take the affix छ्; as आहिच्छत्रः, कान्यकुब्जः.
Why do we say of 'Eastern country'? Because this rule does not apply to a word like देवदत्तः which is the name of a town in the Punjab, which is not an eastern country. Its secondary derivative will be दैवदत्तः, born in Devadatta.
Why do we say 'country'? The rule will not apply to rivers &c. Thus from गोमति we have गौमताः 'fish born in the river Gomati.',