Some essay questions give you special instructions, like using a certain number of examples or discussing a particular event. These instructions set limits for your research, and help you to structure and plan your essay.
Here are a few examples of essay questions with clear limits:
'List one stated objective of the United Nations.'
'Discuss your personal points of view about the meanings and messages of at least two artworks you have studied this year [...]'
'Identify and explain three ways in which gold changed or affected people's visions about the future of Victoria.'
In this question it's made clear you need to pick three ways gold changed people's lives. The subject-specific keywords give you:
- an historical period (gold rush in Victoria - 1850s)
- a place (Victoria, specifically places affected by the gold rush, like Ballarat, Melbourne etc.)
- a topic (how the gold rush changed people's ideas about their future).
Limits are there to make sure you don't spend too long on your essay, or write too much or too little. If your essay question doesn't give you defined limits, come up with your own and make them clear in your introduction.