What is sentence case
We only capitalise the first word in each sentence. This is called sentence case.
This applies to all:
headings
navigation hyperlinks
subsection headers
buttons
The only exception is when we use a proper noun, such as the name of an organisation.
Why we use sentence case
We use sentence case because sentences are easier to scan without capital letters.
From user research we know that capital letters, and particularly BLOCK CAPITALS, are hard to read.
Heading
Subheading
Summary page header
Button
Exception to the rule: proper nouns
Here's an exception to sentence case for the name of an organisation and abbreviation.