Rule 2 states: "We also use a/an to refer to something unspecified, e.g. not particular or specific. ...the shop in the new market is not specific as there are many shops in the new market. So, article has been used before it."
Class 6 › English Grammar and Composition › Chapter 3: Articles: a, an, the › Topic: Rules for 'a' before consonants and 'an' before vowels
Aligned to the NCTB national curriculum.