The article "an" or "a", you use when you're talking about like either a general category of items or

冠词中，“an”或“a”，是当你谈论那些一般的或者可选择的条目时用的，

经常犯的失误 - SpeakingMax英语口语达人

As an illustrator did he have the passion and individual vision that put a satirist such as Daumier (see article) into the category of great art?

ECONOMIST: Exile on Main Street | The

An important category of people, already huge in the rich world and soon to grow in developing countries (see article), consists of elderly people who will never be well, but have no idea when they will die.

ECONOMIST: End-of-life care