And if you are interested in the internals of UDF pragma, I recommend this blog post by Kamil Stawiarski.

The UDF pragma tells the compiler that the PL/SQL unit is a user defined function that is used primarily in SQL statements, which might improve its performance. Dani Schnider showed in this blog post that the performance can be improved by avoiding context switches by using UDF pragma. The difference to a DETERMINISTIC function is that the code is always executed, regardless of the parameter values. From a functional point of view the results for a non-deterministic version of APP_DATE are the same with and without the UDF pragma.