* In the title: Absolutely capitalize the main subject. For example: "The Impact of Social Media on Teenagers" or "Analysis of Global Climate Change."
* In a heading/subheading: Capitalize the main words (like in a title).
* In the body of the report: This is where it gets more flexible. While you should be consistent, you might not always capitalize the subject throughout if it's a long, frequently repeated phrase. The surrounding context and style guide will dictate this. For example, if the report is about "the effects of climate change," you might not capitalize "the effects of climate change" every single time you mention it. Using the consistent term "climate change" in lowercase is usually sufficient for readability.
* Style guides: Always defer to the specific style guide your report follows (e.g., APA, MLA, Chicago). These guides provide specific capitalization rules that will override general guidelines.
In short, while it's usually best practice to capitalize the subject in titles and headings, consistency and adherence to style guides are paramount in the body of the report.