Final answer:
The top user refers to the most active or highest reputation user. Top subject by user limit=3 means subjects organized based on a specific user's contribution.
Step-by-step explanation:
The term top user refers to the user who has the highest level of activity or contribution on a platform or website. In this context, it means the user who has posted the most or has the highest reputation.
On the other hand, top subject by user limit=3 means that the subjects are organized based on the contribution of a specific user. It selects the top subjects in which a user has made the most posts or has contributed the most.