Use title case for headers, titles, and link titles.
Use sentence case for items in bulleted and numbered lists.
For companies, organizations, software, and similar things, use the capitalization of the project.
UNIX was first developed in the 1970s. systemd runs as PID 1.
Match the captalization of commands, even at the beginning of sentences and when otherwise using title case.
How to Check File System Consistency with fsck
For capitalization of DigitalOcean products, see the DigitalOcean products page of this style guide.