The capital letter at the beginning of a sentence lets the reader know that this is where a new sentence starts. The first word of a sentence should always be (first letter) capitalised. Sentences end with a full stop.
Capital letters are also applied to identify names or proper nouns. That is to say, the name of something specific. Writers should be aware not to confuse a ‘common noun’ with a ‘proper noun’. Common nouns do not use capital letters because they are generic or non-specific. For example, ‘country’ as opposed to Australia or ‘park’ and the proper noun (or name) Royal Botanic Gardens.
Headings or titles serve as a naming convention for the content to follow. All words in a heading or title should be capitalised unless they are conjunctions (e.g. and), articles (e.g. the) or prepositions (e.g. to). The first and last word of a heading or title are always capitalised.
Subheadings can be structured as a standard sentence, with only the first word capitalised. However, these can also follow heading or title guidelines if used to establish a type of naming convention for content.