In the following contexts, are header and heading interchangable or is one preferred/correct over the other ?
- the top of a column in an Excel spreadsheet
- the top of an email message (To: CC: BC:)
- the top of a web browser where the address of a website is shown
According to Merriam-Webster,
5b - information (such as a page number) printed or placed at the top of each page of a document (as opposed to footer)
1a - something that forms or serves as a head especially : an inscription, headline, or title standing at the top or beginning (as of a letter or chapter)
Unfortunately, most dictionaries do not have a definition, for either word, for these contexts.