This brief post highlights that when using the Simplified UI any log messages you use in your groovy scripts will be written to the log for the container within which you are accessing the Simplified UI pages (usually Sales or Marketing).
The two screenshots below (and this video) give an example, where the groovy exists in the Common application associated with a custom formula field on the Account standard object, however to get the logging you need to set it on and query it using the Sales application.
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Because the Simplified UI was accessed through the sales container, the respective log messages are found under the Sales application as shown below.
