How do you define a citrus fruit from a fruit?

The Wording is not clear.

• All citrus fruits are fruits
• All fruits are not necessarily citrus

Guessing that you want to know which fruits are citrus it is based on acidity (citric acid).

Some examples of fruits that have HIGH (many fruits have citric acid, only those of high content are considered citrus fruits)levels of citric acid, ther fore citrus fruits:

• Orange
• Lemon
• Lime
• Grapefruit