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Within normal usage it should not be necessary to use the button Invalidate caches now. If the need arises anyway, don’t despair 
: All it will trigger is for all of your users' browser to freshly load and cache the field mappings defined for the user’s project(s).
If you are interested in how it works in more detail: Open the browser developer console and check the local storage for keys starting with JFIELDS. For Chrome, more information on the local storage is available on https://developer.chrome.com/docs/devtools/storage/localstorage/?utm_source=devtools (external link).