WP_MEMORY_LIMIT is max memory your WordPress will use while generating a http response, I mean when you a hit a URL, WordPress content has to be generated (html, css, js etc) before it is delivered, Hence any Server side Rendering/creating of the webpage shouldn't use more than WP_MRMORY_LIMITWP_MEMORY_LIMIT(I have this set to 64MB.) so if 4 simultaneous URL hits are received on WordPress server then 256MB of memory will be consumed(64x4). Once the page is rendered/ generated and sent over http response this memory is freed.