Firstly, a novel is a work of art, and as such, the author must make choices about what to include and what to exclude in order to create a coherent and meaningful narrative. This means being selective about the characters, events, and settings that are included in the story.
Secondly, a novelist must consider the length of their novel and the amount of time they have to write it. This means that they need to be selective about the amount of detail they include in the story, and to focus on the most important moments and events.
Thirdly, a novelist must consider the audience for their novel and what they will find interesting and engaging. This means being selective about the subject matter and ensuring that it is relevant to the interests and experiences of the intended audience.
Finally, a novelist must consider the overall structure and unity of their novel. This means being selective about the subplots and side stories that are included, and ensuring that they contribute to the main plot and themes of the story.
In short, a novelist must be selective about their subject in order to create a coherent, meaningful, and engaging work of art that will appeal to their intended audience.