When pronounced as individual letters, we say "an XL" since the letter X begins with a vowel sound.

It’s an X-ray. Since the letter X begins with a vowel sound, we use the article "an" before the word X-ray (a type of radiation that can go through many solid objects).

It’s an X. Since the pronunciation of the individual letter "X" begins with a vowel sound, we use the indefinite article "an".

It’s a URL. The abbreviation URL begins with a vowel, but it has a consonant sound ("you").

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”.