WP_MEMORY_LIMIT vs WP_MAX_MEMORY_LIMIT

I always thought WP_MAX_MEMORY_LIMIT related to back end memory and the setting lives in wp-includes/default-constants.php. I have WP_MEMORY_LIMIT defined at 64M in the wp-config.php file and WP_MAX_MEMORY_LIMIT set at the default 256M, but I still get intermittent memory exhausted errors when editing.
I’m looking for a clearer explanation of the roles of these limits so I can better manage this site.
And is a change to WP_MAX_MEMORY_LIMIT wise, and if so, would it be made in wp-includes/default-constants.php or added to wp-config?