With the release of the Dev Notes for the upcoming WordPress 6.3 release, there are many exciting features that have not yet been highlighted. The new development mode, activated by declaring the WP_DEVELOPMENT_MODE constant, will initially be especially useful for theme developers.
"The development mode configured on a site defines the type of development work the site is used forsaid Felix Arntz, a lead developer of Google-sponsored WordPress. This mode is not recommended for production sites.
Possible values for the WP_DEVELOPMENT_MODE constant include “core”, “plugin”, “theme”, “all”, or an empty string (which is the default value). The value "all" is applicable to sites where all three aspects could be changed, such as a client website under construction.
There are currently only a few use cases in WordPress Core that are determined by how you are developing, but that is likely to increase in the future. Most usage today is for theme.json cache
Since the cache is typically only invalidated when the theme is updated, it can become burdensome for developers who are actively editing the theme.json file and have to manually invalidate it to see their changes. This caching functionality is bypassed when the value is set to "theme".
While the WP_ENVIRONMENT_TYPE constant looks similar to the new development mode, it specifically indicates whether the environment is development, staging, or production, but it doesn't specify what kind of development is being done.
It is likely that you will only use the WP_DEVELOPMENT_MODE constant on a site where WP_DEBUG is enabled and WP_ENVIRONMENT_TYPE is 'development' or 'local', as it is not recommended to develop directly against staging or production environments
Arntz pointed out.
For more details on when and how to use Developer Mode, and for code examples to check if development mode is enabled on a site, developers can refer to the developer note posted on the blog make.wordpress.org/core.