As a companion to WP_CLI_EARLY_REQUIRE but doesn't include the files until after wp has been through the boostrap process. This is the same as using --require from the command line, or require in a yaml config file. The purpose for another way of requiring a file is for use in codecov testing, where it otherwise is not practical to conditionally include a php file at normal runtime using the existing methods in a testing environment.

See wp-cli/wp-cli-tests#241 for more conversation. This is what #6065 was meant to address, but it didn't end up working out for this specific case since we can't include our codcov generate files that early on before wp has been through bootstrap