It’s a UNESCO. For example, we say a UNESCO World Heritage Site, a UNESCO project, a UNESCO study, etc. Since the word UNESCO begins with a consonant sound ("you"), we use the article a. The reef has been listed a UNESCO World Heritage Site. The reef has

It’s a UN. When speaking about the United Nations, the letter "U" has a consonant sound (like "you"), and therefore takes the indefinite article "a".

It’s a UK. When speaking about the United Kingdom, the letter U has a consonant sound (like "you"), and therefore takes the article "a".

It’s a UID. Since the abbreviation UID (Unique Identifier) begins with a consonant sound (the letter "u" sounds like “you”), we use the article “a”.

It’s a UFO. Since the abbreviation UFO (Unidentified Flying Object) begins with a consonant sound (a “you” sound), we use the indefinite article “a”.

It’s a U. When pronounced as an individual letter, the vowel "U" has a consonant sound (it sounds like "you"), and takes the article "a".

It’s an SVG. Since the abbreviation SVG (Search Engine Optimization) begins with a vowel sound ("ess"), we use the indefinite article "an".

It’s an SEO. The abbreviation SEO is usually pronounced as individual letters. Since the consonant "S" begins with a vowel sound ("ess"), we use the article "an".

It’s an SUV. Since the first letter (S) begins with a vowel sound ("ess"), we use the indefinite article "an".