Custom subject variable name discrepancy - what does {} mean?

https://github.com/apache/infrastructure-asfyaml/blob/main/README.md#custom-subject-lines-for-github-events includes a list of possible subject variables.

About half the variables are enclosed in `{}` - is that intentional, and if so, how do they differ from variables not so enclosed?
e.g. what is the difference between `title` and `{action}` ?

Start with README.md at the “Custom subject lines for GitHub events” section and compare the listed variables, including title and {action}. Check the related configuration or schema references mentioned there to determine whether braces have a defined meaning. Done means documenting the distinction, or stating clearly that the notation is inconsistent, with corrected examples.

Written by the indexing model from the issue text.