When do I need to capitalize a job title?
1. On your resume.
It's standard practice to use capital letters for your job titles on your resume or curriculum vitae (CV); it would look strange if you didn't.
For example, it should look something like this:
Head of Marketing, Example Company, New York, NY 2019 - present
Director of Sales, Another Company, 2011 - 2018
2. In your cover letter.
This isn't as hard and fast as No. 1, but for the most part, it's standard practice to use capital letters when referring to the specific role (say, Vice President of Finance).
However, if your cover letter doesn't explicitly state the role you're applying for (and it doesn't have to, that could be in the email subject line) and you just mention joining the team, you won't have to capitalize that (example: "I'm excited to discuss how I'd contribute to the sales team," not "I'm excited to discuss how I'd contribute to the Sales Team").
3. In the job listing.
Most (if not all that we can think of) job listings use title case for jobs. When you peruse a job board, you'll notice that all job titles are capitalized.
Do you use capital letters for job titles?
Yes, it depends on the context; see the explanations above!
When should capital letters be used?
In general, when they're required.
Proper nouns (names, places, days of the week, etc) usually require capital letters. Whether the title of a story or a subheading require title case or sentence case will depend on what style you're following. Follow the guidelines outlined in AP, MLA, APA, Chicago or whatever standard you're following.
If what you're writing isn't following a prescribed format, default to the style the publication or company or example materials use.
Is a job title a proper noun?
In short, it can be. For example, the CEO or the President is a proper noun. However, in AP style, which you can see in almost all newspapers, when a title is used without a name it's not capitalized; for instance, "On Thursday, the president spoke to the press." However, when the job title is used with a name, it is capitalized: suchs as "Chief Financial Officer Jeffrey S. DeWitt."
How do you capitalize titles?
Again, this depends. Academic titling will follow the prescribed standards outlined in style guides such as APA, Chicago or MLA. Media generally defaults to AP, but within that, most companies generally have in-house style guides that describe how titles should be capitalized. In general, all principal words, verbs, names, pronouns, adjectives and adverbs are capped while prepositions and conjunctions are not (if they're less than four words).